A charging device for a battery of a vehicle includes an insulation resistance detecting circuit, a charging circuit, a leak current detecting circuit and a decoupling circuit. The insulation resistance detecting circuit includes a coupling capacitor, and is disposed between the battery and a vehicle body for detecting an insulation resistance between the battery and the vehicle body. The charging circuit converts an alternating current supplied from an alternating current source into a direct current and charges the battery in a state without insulting the input terminal and the output terminal and in a state where the vehicle body is coupled to an earth. The leak current detecting circuit detects a leak current between the charging circuit and the earth. The decoupling circuit decouples the insulation resistance detecting circuit from one of the battery and the vehicle body during a charging of the battery.