To provide a motor drive unit, which enables to prevent overshooting of supply voltage to an inverter by DC current which is caused when an instantaneous interruption of power supply occurs.

In this motor drive unit when it is provided with an active filter 13 operating normal, an abnormal voltage drop Vp is detected by making reference to a first reference voltage V1. When an abnormal drop is detected, a feeder line is interrupted by a circuit breaker 18, and a breakdown of circuit parts caused from an overshoot in voltage Vp due to an instantaneous interruption of power or the like is prevented. When the motor drive unit is not provided with the active filter 13 or even if provided it does not operate normally, abnormal drop of the voltage Vp is detected by making reference to a second reference voltage V2. When an abnormal voltage drop is detected, the feeder line is interrupted by the circuit breaker 18, and an operation trouble of the motor 21 caused from a supply power shortage to the inverter 17 is prevented.