To provide a method for detecting dynamic brake circuit failure, which detects a failure with a relay and a rectifying diode or the like constituting the dynamic brake circuit, while a motor is not driven.

A motor controller includes: the dynamic brake circuit 23 composed of a three-phase full wave rectifying circuit 22, the relay 17 and a braking resistor 18, an inverter part 21, a current detection means 19 for detecting a current flowing through the braking resistor 18, and a contactor 25 installed on a power line of the motor 20. When the motor 20 is stopped and enters a non-conductive state, the dynamic brake circuit 23 is turned on by opening the contactor 25, one of semiconductor switching elements of the upper side arm of an inverter part 21 is turned on, one of the semiconductor switching elements of the lower side arm is turned on, and a failure in the dynamic brake circuit 23 is detected based on a current value detected by the current detection means 19.