PURPOSE: To easily grasp an operating state of a motor group connected with many motors and to simply discover a motor route in which a trouble occurs.

CONSTITUTION: A motor monitor 3 is branched to a connecting line for connecting a plurality of motors 1 connected in parallel to an inverter 2 through a current transformer CT. The monitor 3 has a detector 4 connected to the transformer CT, an arithmetic processor CPU, and a monitoring unit 8. The detector 4 has a current detector 5 for detecting a sum of currents flowing to the motor, and a slope detector 6 for detecting a slope of a starting current of the motor. The processor CPU compares a previously stored current and its varied reference value with actually detected one to calculate number of connected motors. The unit 8 indicates calculated number, detected current value, etc.