A ground fault detection device configured to be connected to a non-grounded battery and to calculate an insulation resistance of a system with the battery provided therein, comprising: a detection capacitor (C1) configured to be operated as a flying capacitor; a switch group (S1-S4) for switching between a first charging path, a second charging path, a third charging path, and a measurement path, the first charging path connecting the battery and the detection capacitor (C1), wherein one of the second charging path and the third charging path is a charging path connecting the battery , a negative side insulation resistance (RLn) and the detection capacitor (C1) and another of the second charging path and the third charging path is a charging path connecting the battery , a positive side insulation resistance (RLp) and the detection capacitor (C1), wherein the negative side insulation resistance (RLn) is an insulation resistance for a negative side line of the battery to a ground and the positive side insulation resistance (RLp) is an insulation resistance for a positive side line of the battery to the ground, wherein the measurement path is configured for measuring a charging voltage of the detection capacitor (C1); and a controller configured to control the switch group (S1-S4) and to calculate the insulation resistance based on a charging voltage measured value of the detection capacitor (C1) which exists after charging each of the charging paths, characterized by that after measurement of the charging voltage of the second charging path, the controller is configured to cause the switch group (S1-S4) to temporarily switch to the third charging path before switching to the first charging path. a detection capacitor (C1) configured to be operated as a flying capacitor; a switch group (S1-S4) for switching between a first charging path, a second charging path, a third charging path, and a measurement path, the first charging path connecting the battery and the detection capacitor (C1), wherein one of the second charging path and the third charging path is a charging path connecting the battery , a negative side insulation resistance (RLn) and the detection capacitor (C1) and another of the second charging path and the third charging path is a charging path connecting the battery , a positive side insulation resistance (RLp) and the detection capacitor (C1), wherein the negative side insulation resistance (RLn) is an insulation resistance for a negative side line of the battery to a ground and the positive side insulation resistance (RLp) is an insulation resistance for a positive side line of the battery to the ground, wherein the measurement path is configured for measuring a charging voltage of the detection capacitor (C1); and a controller configured to control the switch group (S1-S4) and to calculate the insulation resistance based on a charging voltage measured value of the detection capacitor (C1) which exists after charging each of the charging paths, characterized by that after measurement of the charging voltage of the second charging path, the controller is configured to cause the switch group (S1-S4) to temporarily switch to the third charging path before switching to the first charging path. <EOS>