An anomaly detecting method in a servo system that is capable of quickly detecting the occurrence of an anomaly even in a low revolution range is disclosed. A simulator (14) finds a position feedback (PAS) by integrating a speed instruction (Vcs) which is equal to the product of a position loop gain (PG) and an estimated position deflection (Ers) that is obtained by subtracting the position feedback (PAS) from a movement instruction (Mc) while the servo control for the servo motor is executed by the digital servo control units (11 to 13) according to a torque instruction (Tc) calculated based on the speed feedback (VA) and a speed instruction (Vc) that corresponds to a practical position deviation (Er). An extraordinary error detecting unit (15) compares a predetermined value (As) with the error (Er - Ers) between the practical position deviation and an estimated position deviation. In case any anomaly has occurred, the practical position deviation increases but there is no substantial change in the estimated position deviation generated by the simulator that approximately carries out normal servo control, whereby a difference between them reaches a predetermined value that indicates the occurrence of an anomaly and alarm is set off.