PURPOSE: To allow detection of overload conditions of a motor at any point of time from immediately after start to steady operation thereof by providing means for adding a predetermined coefficient to a detected current value of motor or multiplying the detected current value of motor by a predetermined coefficient to operate a threshold value.

CONSTITUTION: When a start switch BS1 is depressed while setting a memory mode by means of a memory/reproduction switch 12 after confirmation that a machine 4 to be driven through a motor M is normally loaded, the motor M is started and a value S0 detected through a motor current detecting means 8 is stored in a memory 22. The motor M is eventually stopped by depressing a stop switch BS2. When the motor M is started again by depressing the switch SB1 while setting a reproduction mode by means of the switch 12, a threshold value S3 is operated through an operating means 26 by multiplying the detection values S0, stored in the memory 22, by a predetermined coefficient (k). A decision means 28 then compares the threshold value S3 sequentially with the value S0 detected through the detecting means 8 and delivers an interruption signal S4 when a state S0≥3, sustains longer than a predetermined time interval.