健康保険の入門 — 国保と社保の違い
日本は「国民皆保険」の制度を採用しており、原則としてすべての居住者が何らかの公的医療保険に加入する義務があります。主な制度は2つあり、一つは会社員とその家族が加入する「健康保険（社保）」、もう一つは自営業者・無職・退職者などが加入する「国民健康保険（国保）」です。
Japan adopts a "universal health coverage" system, in which in principle all residents are obligated to enroll in some form of public medical insurance. There are two main schemes: "Employees' Health Insurance" (shaho) for company employees and their families, and "National Health Insurance" (kokuho) for the self-employed, unemployed, and retirees.
社保（健康保険）の基本
健康保険（社保）は、原則として週30時間以上勤務する正社員や、一定条件を満たす短時間労働者（週20時間以上・月額賃金88,000円以上・従業員51人以上の企業など）が加入します。運営主体は2種類：中小企業を中心とした「全国健康保険協会（協会けんぽ）」と、大企業が独自に設立する「健康保険組合（組合健保）」です。
Employees' Health Insurance (shaho) generally covers regular employees working 30+ hours a week, plus part-time workers meeting certain conditions (20+ hours weekly, monthly wage of 88,000 yen or more, employer with 51+ employees). There are two operating bodies: Japan Health Insurance Association (Kyokai Kenpo), which mainly covers SMEs, and corporate union health insurance (Kumiai Kenpo), independently established by large corporations.
保険料は「標準報酬月額」（毎月の給与を50等級に区分した額）に保険料率（協会けんぽは都道府県ごとに異なり、全国平均10%前後）を掛けて算出され、労使が折半します。つまり従業員は実際の保険料の半分（約5%）のみを負担します。賞与（ボーナス）からも「標準賞与額」をベースに保険料が徴収されます。
Premiums are calculated by multiplying the "standard monthly remuneration" (the monthly salary divided into 50 grade brackets) by the premium rate (Kyokai Kenpo rates differ by prefecture, averaging around 10% nationally) and split evenly between employer and employee. Thus the employee bears only half (around 5%) of the actual premium. Premiums are also collected from bonuses based on the "standard bonus amount."
国保（国民健康保険）の基本
国民健康保険（国保）は、市町村（および都道府県）が運営する制度で、社保に加入していない人が対象です。具体的には個人事業主・フリーランス・農業従事者・退職者（任意継続を利用しない場合）・無職の人・非正規雇用で社保の加入要件を満たさない人などです。
National Health Insurance (kokuho) is operated by municipalities (and prefectures) and covers people not enrolled in employees' insurance. Specifically, this includes sole proprietors, freelancers, agricultural workers, retirees (when not using voluntary continuation), the unemployed, and non-regular workers who do not meet the requirements for shaho enrollment.
国保の保険料は、前年の所得と世帯の加入人数に基づき、市町村ごとに計算されます。一般には①所得割、②均等割、市町村によっては③平等割・④資産割を組み合わせて算出されます。年間上限は医療分・後期高齢支援分・介護分を合計して年102万円程度（年度・市町村により変動）。全額が本人負担（折半なし）となるため、同等の年収でも社保より割高に感じられることが多いです。
Kokuho premiums are calculated by each municipality based on the previous year's income and the number of enrolled household members. Typically the calculation combines (1) an income-based portion, (2) a per-person portion, and depending on the municipality (3) a per-household portion and (4) an asset-based portion. The annual cap, totaling medical / later-stage elderly support / long-term care portions, is approximately 1.02 million yen (varies by year and municipality). Since the entire amount is paid by the individual (no 50/50 split), it often feels relatively expensive compared to shaho at equivalent income levels.
医療費の自己負担割合と給付
公的医療保険の給付は原則として、医療費の70%を保険が負担し、残り30%を患者が窓口で支払います。これは社保・国保とも共通です。ただし年齢により次のように異なります。未就学児（6歳に達する年度の年度末まで）は自己負担20%。70歳〜74歳は原則20%（現役並み所得者は30%）。75歳以上は後期高齢者医療制度に移行し原則10%（一定所得以上は20%・現役並み所得者は30%）。
Public medical insurance benefits in principle cover 70% of medical costs, with the patient paying the remaining 30% at the counter. This is the same for shaho and kokuho. However, age-based differences apply as follows. Pre-school children (until the end of the fiscal year in which they turn 6) bear 20%. Ages 70-74 in principle 20% (active-equivalent earners pay 30%). Age 75 and over transitions to the late-stage elderly medical system, in principle paying 10% (those at certain income levels pay 20%, active-equivalent earners pay 30%).
高額療養費制度
1ヶ月（月の初日から末日まで）の医療費自己負担額が一定の上限を超えた場合、その超過分が後日還付される制度が高額療養費制度です。70歳未満・一般所得区分（年収約370万〜770万円）の人の計算式は「80,100円 + （総医療費 − 267,000円）×1%」です。例えば、総医療費が100万円（窓口負担30万円）の場合、自己負担上限は約87,430円となり、差額約21万円が還付されます。
The high-cost medical expense system refunds the excess portion when monthly out-of-pocket medical costs (from the first to the last day of the month) exceed a certain cap. For people under 70 in the general income bracket (annual income approx. 3.7M to 7.7M yen), the formula is: "80,100 yen + (total medical cost - 267,000 yen) × 1%." For example, if total medical costs are 1 million yen (counter payment 300,000 yen), the out-of-pocket cap is approximately 87,430 yen, and the difference of approximately 210,000 yen is refunded.
事前に加入している健康保険（協会けんぽ・組合健保・市町村）に「限度額適用認定証」を申請しておけば、医療機関の窓口で最初から自己負担上限額までの支払いで済みます。特に入院や高額な治療を受ける際に非常に重要な制度です。
By applying in advance for the "Limit Amount Application Certificate" from your health insurance (Kyokai Kenpo, Kumiai Kenpo, or municipality), you can pay only up to the out-of-pocket cap amount at the medical institution's counter from the start. This is an extremely important system, especially when hospitalized or receiving expensive treatment.
扶養（被扶養者）制度
社保には「被扶養者」制度があり、一定の要件を満たす家族は追加保険料なしで同じ保険に加入できます。主な要件は、①年間収入130万円未満（60歳以上または障害者は180万円未満）、②被保険者（本人）の年間収入の2分の1未満（同居要件に応じて異なる）です。配偶者・子・父母・孫・兄弟姉妹などが対象です。
Shaho has a "dependent" (hifuyousha) system, allowing family members meeting certain requirements to enroll in the same insurance without additional premiums. Main requirements: (1) annual income under 1.3 million yen (under 1.8 million yen for those 60+ or disabled), (2) less than half of the insured person's annual income (depending on cohabitation requirements). Eligible relatives include spouse, children, parents, grandchildren, and siblings.
一方、国保には「扶養」の概念が存在しません。世帯全員がそれぞれ被保険者となり、各人の所得に応じた保険料（均等割を含む）が算出され、世帯主が一括納付します。家族が多いと均等割が積み重なり、割高になりがちです。
On the other hand, kokuho has no "dependent" concept. Every household member is an insured person, and premiums (including a per-person portion) are calculated by each person's income, with the household head paying in bulk. With many family members, the per-person portions accumulate, tending to make it relatively expensive.
転職・退職時の切り替え
退職・転職の際は資格喪失から14日以内に次の手続きを行う必要があります。選択肢は3つ：①転職先の社保に加入（空白期間なしの転職が理想）、②任意継続被保険者制度：退職前に2ヶ月以上加入していた人は最長2年間同じ健康保険を継続できる（ただし保険料は全額本人負担）、③国保に加入（市町村役所に退職を証明する書類を持参）。
When retiring or changing jobs, you must complete the next procedure within 14 days of losing eligibility. There are 3 options: (1) enroll in the new employer's shaho (job changes without a gap are ideal), (2) Voluntary Continuation Insured Person system: those enrolled for 2+ months before retirement can continue the same health insurance for up to 2 years (but premiums become fully self-paid), (3) enroll in kokuho (bring documents proving retirement to the municipal office).