PURPOSE: To reliably prevent breakage of a tool, in a surplus load detecting system for a CNC to detect a surplus load exerted on a tool and a machine element during a machining work.

CONSTITUTION: Disturbance load torque Yz exerted on a servo motor 63 is estimated by a disturbance load torque estimating means 1 based on a speed signal X1z for a servo motor 63 and a current command value U1z to a servo motor 63. A rate of change α the estimated disturbance load torque Yz is computed at intervals of a sampling period by a rate of change computing means 2. When the rate of change α of the disturbance load torque Yz exceeds a lowermost reference value α1, rotation of the servo motor 63 is decelerated by a given amount M1 by a speed command value control means 3. Further, when the rate of change α exceeds a reference value α2 higher than the reference value α1, rotation of the servo motor 63 is decelerated by a given amount M2 being, for example, two times as large as the given amount M1. Moreover, when the rate of change α exceeds a highmost reference value α3, the rotation of the servo motor 63 is stopped.