diff --git a/backend/app/payroll/bonus/service.py b/backend/app/payroll/bonus/service.py
index 5228368..bcfd87d 100644
--- a/backend/app/payroll/bonus/service.py
+++ b/backend/app/payroll/bonus/service.py
@@ -4,7 +4,7 @@
 import json
 import os
 import functools
-from decimal import Decimal, ROUND_HALF_UP
+from decimal import Decimal, ROUND_HALF_UP, ROUND_DOWN
 from datetime import date
 from typing import Dict, Any
 from ...core.database import get_connection
@@ -203,7 +203,7 @@ class BonusCalculationService:
         if health_insurance_rate:
             health_insurance = (
                 health_standard_bonus * Decimal(str(health_insurance_rate["employee_rate"]))
-            ).quantize(Decimal("1"), rounding=ROUND_HALF_UP)
+            ).quantize(Decimal("1"), rounding=ROUND_DOWN)
         
         # 介護保険（40歳以上65歳未満が対象）
         care_insurance = Decimal("0")
@@ -212,7 +212,7 @@ class BonusCalculationService:
             if care_insurance_rate:
                 care_insurance = (
                     health_standard_bonus * Decimal(str(care_insurance_rate["employee_rate"]))
-                ).quantize(Decimal("1"), rounding=ROUND_HALF_UP)
+                ).quantize(Decimal("1"), rounding=ROUND_DOWN)
         
         # 厚生年金
         pension_insurance_rate = BonusCalculationService.get_insurance_rate("厚生年金", payment_date)
@@ -220,7 +220,7 @@ class BonusCalculationService:
         if pension_insurance_rate:
             pension_insurance = (
                 pension_standard_bonus * Decimal(str(pension_insurance_rate["employee_rate"]))
-            ).quantize(Decimal("1"), rounding=ROUND_HALF_UP)
+            ).quantize(Decimal("1"), rounding=ROUND_DOWN)
         
         # 雇用保険
         employment_insurance_rate = BonusCalculationService.get_insurance_rate("雇用保険", payment_date)
@@ -228,7 +228,7 @@ class BonusCalculationService:
         if employment_insurance_rate:
             employment_insurance = (
                 total_bonus * Decimal(str(employment_insurance_rate["employee_rate"]))
-            ).quantize(Decimal("1"), rounding=ROUND_HALF_UP)
+            ).quantize(Decimal("1"), rounding=ROUND_DOWN)
         
         # 子ども・子育て支援金（令和8年5月支給分以降のみ適用）
         # 賞与は bonus_month でなく実際の支給日（payment_date）で判定する
@@ -241,7 +241,7 @@ class BonusCalculationService:
             if child_support_rate:
                 child_support = (
                     total_bonus * Decimal(str(child_support_rate["employee_rate"]))
-                ).quantize(Decimal("1"), rounding=ROUND_HALF_UP)
+                ).quantize(Decimal("1"), rounding=ROUND_DOWN)
                 print(f"[DEBUG] 賞与子育て支援金: 賞与総額={total_bonus} × 料率={child_support_rate['employee_rate']} = {child_support}")
         
         # 所得税の計算（賞与特有の計算）
diff --git a/backend/app/payroll/calculation/service.py b/backend/app/payroll/calculation/service.py
index 6c7d2cd..fbeeb3e 100644
--- a/backend/app/payroll/calculation/service.py
+++ b/backend/app/payroll/calculation/service.py
@@ -1,7 +1,7 @@
 """
 給与計算サービス (Payroll Calculation Service)
 """
-from decimal import Decimal, ROUND_HALF_UP
+from decimal import Decimal, ROUND_HALF_UP, ROUND_DOWN
 from datetime import date
 from typing import Dict, Any
 from ...core.database import get_connection
@@ -381,7 +381,7 @@ class PayrollCalculationService:
         if health_insurance_rate:
             health_insurance = (
                 health_standard_salary * Decimal(str(health_insurance_rate["employee_rate"]))
-            ).quantize(Decimal("1"), rounding=ROUND_HALF_UP)
+            ).quantize(Decimal("1"), rounding=ROUND_DOWN)
         elif calc_social_insurance:
             errors.append(f"健康保険料率が見つかりません（年度: {payment_date.year}）")
         
@@ -392,7 +392,7 @@ class PayrollCalculationService:
             if care_insurance_rate:
                 care_insurance = (
                     health_standard_salary * Decimal(str(care_insurance_rate["employee_rate"]))
-                ).quantize(Decimal("1"), rounding=ROUND_HALF_UP)
+                ).quantize(Decimal("1"), rounding=ROUND_DOWN)
                 print(f"[DEBUG] 介護保険計算: {health_standard_salary} × {care_insurance_rate['employee_rate']}% = {care_insurance}")
             else:
                 errors.append(f"介護保険料率が見つかりません（年度: {payment_date.year}）") if calc_social_insurance else None
@@ -403,7 +403,7 @@ class PayrollCalculationService:
         if pension_insurance_rate:
             pension_insurance = (
                 pension_standard_salary * Decimal(str(pension_insurance_rate["employee_rate"]))
-            ).quantize(Decimal("1"), rounding=ROUND_HALF_UP)
+            ).quantize(Decimal("1"), rounding=ROUND_DOWN)
             print(f"[DEBUG] 厚生年金計算: {pension_standard_salary} × {pension_insurance_rate['employee_rate']}% = {pension_insurance}")
         elif calc_social_insurance:
             errors.append(f"厚生年金保険料率が見つかりません（年度: {payment_date.year}）")
@@ -423,7 +423,7 @@ class PayrollCalculationService:
             if child_support_rate:
                 child_support = (
                     standard_monthly_salary * Decimal(str(child_support_rate["employee_rate"]))
-                ).quantize(Decimal("1"), rounding=ROUND_HALF_UP)
+                ).quantize(Decimal("1"), rounding=ROUND_DOWN)
                 print(f"[DEBUG] 子育て支援金: 標準報酬月額={standard_monthly_salary} × 料率={child_support_rate['employee_rate']} = {child_support}")
 
         # 雇用保険加入対象かどうか確認
@@ -432,7 +432,7 @@ class PayrollCalculationService:
         if is_employment_insurance_eligible and employment_insurance_rate:
             employment_insurance = (
                 total_payment * Decimal(str(employment_insurance_rate["employee_rate"]))
-            ).quantize(Decimal("1"), rounding=ROUND_HALF_UP)
+            ).quantize(Decimal("1"), rounding=ROUND_DOWN)
         elif not is_employment_insurance_eligible:
             print(f"[DEBUG] この従業員は雇用保険非対象です")
         elif not employment_insurance_rate and calc_social_insurance: