PURPOSE: To surely protect a cam mechanism for a tool changing device and the like by precisely judging an overload corresponding to a cam rotation angle.

CONSTITUTION: A numerical control device 21 is provided with an allowable torque constant memory circuit 22, a motor drive circuit 23, a motor current value measuring circuit 24, an overload judging circuit 25 and an alarm generating circuit 26. An allowable torque constant table corresponding to a cam rotation angle is stored in the allowable torque constant memory circuit 22. A motor current value for each cam rotation angle is measured by the motor current value measuring circuit 24. The overload judging circuit 25 judges an overload through comparison between an allowable torque constant and the motor current value and outputs a motor stop command to the motor drive circuit 23 and an alarm command to the alarm generating circuit 26, respectively.