A device for properly detecting the generation of excessive positional deviation in a servo system for a wide range of rotational speed of a servo motor to correctly and quickly determine the overload condition of the servo motor. A central processing unit in the numerical controller may determine that the practical positional deviation read from the axis controller is greater than an allowable maximum value of a positional deviation that is obtained by dividing the product of a maximum value of a pulse distribution amount stored in the record portions of a table in the memory and a predetermined proportional constant by a position loop gain (S4 to S6). It is then determined that an excessive positional deviation has generated in the servo system due to abnormal condition in the servo system, overload of the servo motor, or interference between the mechanical moving part and foreign substance. The servo motor is then stopped and the alarm message is displayed (S7).