A motor driving circuit has a supply line connected to a DC power source, an inverter whose input side is connected to the supply line and whose output side is connected to a motor, a power switch inserted in the supply line for switching the supply line between conducting and cut-off states, a voltage detector for detecting a voltage between the direct-current power source and the power switch, and an insulation resistance detector for first detecting, based on a result of detection by the voltage detector with the power switch in the cut-off state, an insulation resistance on the preceding-stage side of the power switch and subsequently detecting, based on a result of detection by the voltage detector with the power switch in the conducting state, an insulation resistance on the succeeding-stage side of the power switch.