A motor drive control device is provided in which if any abnormality occurs in a drive control circuit, drive command signals the drive control circuit generates are interrupted at once, so that an AC motor can be stopped in safety. A monitor control circuit and drive command signal interruption circuit are provided to the drive control circuit that takes variable-speed control of the AC motor supplied with power from a DC drive power source via a semiconductor bridge circuit. If any abnormality occurs in the drive control circuit, the drive command signals the drive control circuit generates are interrupted at once. When starting operation, the drive control circuit and monitor control circuit cooperate with each other to conduct preliminary check as to whether or not the drive command signal interruption circuit operates normally, base on a predetermined time schedule, and stop the AC motor without fail if any abnormality occurs during operation.