A ground-fault detecting device includes a power source which is electrically insulated from a vehicle body, a pulse signal generator which generates a pulse signal having a high level and a low level which appear repeatedly in a prescribed cycle, a detection resistor which is connected to the pulse signal generator and the power source, a coupling capacitor which is connected to the detecting resistor in series, an integrator which integrates a difference between a first reference voltage and a detection voltage of the pulse signal at a connecting point of the detection resistor and the coupling capacitor over an integration interval, and a ground-fault determinant which judges whether a ground fault has occurred on the basis of an output of the integrator. The integration interval is at least part of a high-level interval and a low-level interval of the pulse signal.