diff --git a/TIASshot/Cameras/GainCalc.cs b/TIASshot/Cameras/GainCalc.cs
index 1ba799a..9854e5d 100644
--- a/TIASshot/Cameras/GainCalc.cs
+++ b/TIASshot/Cameras/GainCalc.cs
@@ -28,7 +28,15 @@
         /// <param name="rangeMax">設定可能な最大値</param>
         /// <returns>範囲内に収めた新しいゲイン設定値</returns>
         internal static int CalcGainValue(int currentValue, float ratio, int rangeMin, int rangeMax) {
-            return (int)(0.5 + currentValue * ratio);
+            // 計測値・目標値がともに 0 のとき比率は NaN になり更新量を決められないため現在値を維持する
+            if (float.IsNaN(ratio)) return currentValue;
+
+            // float→int キャストは範囲外・非有限値（Infinity）で未定義値になるため，
+            // キャスト前に double のまま範囲判定する
+            double value = 0.5 + (double)(currentValue * ratio);
+            if (value < rangeMin) return rangeMin;
+            if (value > rangeMax) return rangeMax;
+            return (int)value;
         }
     }
 }
diff --git a/TIASshot/Cameras/IScam.cs b/TIASshot/Cameras/IScam.cs
index da3b09a..9e125f2 100644
--- a/TIASshot/Cameras/IScam.cs
+++ b/TIASshot/Cameras/IScam.cs
@@ -123,9 +123,18 @@
         /// </summary>
         /// <param name="whitePatch">白パッチの計測値</param>
         protected override void ApplyWhiteBalance(Scalar whitePatch) {
-            _whiteBalanceBlue.Value = (int)(0.5 + _whiteBalanceBlue.Value * GetRatio((float)whitePatch.Val0, Config.GetFloat("Calib/Reference/B")));
-            _whiteBalanceGreen.Value = (int)(0.5 + _whiteBalanceGreen.Value * GetRatio((float)whitePatch.Val1, Config.GetFloat("Calib/Reference/G")));
-            _whiteBalanceRed.Value = (int)(0.5 + _whiteBalanceRed.Value * GetRatio((float)whitePatch.Val2, Config.GetFloat("Calib/Reference/R")));
+            _whiteBalanceBlue.Value = GainCalc.CalcGainValue(
+                _whiteBalanceBlue.Value,
+                GetRatio((float)whitePatch.Val0, Config.GetFloat("Calib/Reference/B")),
+                _whiteBalanceBlue.RangeMin, _whiteBalanceBlue.RangeMax);
+            _whiteBalanceGreen.Value = GainCalc.CalcGainValue(
+                _whiteBalanceGreen.Value,
+                GetRatio((float)whitePatch.Val1, Config.GetFloat("Calib/Reference/G")),
+                _whiteBalanceGreen.RangeMin, _whiteBalanceGreen.RangeMax);
+            _whiteBalanceRed.Value = GainCalc.CalcGainValue(
+                _whiteBalanceRed.Value,
+                GetRatio((float)whitePatch.Val2, Config.GetFloat("Calib/Reference/R")),
+                _whiteBalanceRed.RangeMin, _whiteBalanceRed.RangeMax);
         }
 
         /// <summary>