If a bus current a bus current detector 11 detects exceeds a current cutoff value, an overcurrent detector 12 supplies a gate driver 14 with an open signal for turning off switching elements 5-10 to break the current to a motor 1. Besides, when the overcurrent detector 12 detects an overcurrent, a feedback controller 15 provides the gate driver 14 with a short-circuit braking instruction to apply short-circuit braking after breaking the power supply, and to restart PWM control from a state in which the short-circuit braking is being applied and the duty ratio command value is reset at 0.