PURPOSE: To provide a fault diagnostic device for the servo control system which makes it easy to demarcate fault contents.

CONSTITUTION: The fault diagnostic device for the servo control system which drives the actuator of a hydraulic cylinder, etc., with the servo output from a servo driver and also feeds the output of a sensor detecting the driving quantity of the actuator back to the servo driver from the actuator is constituted by inserting a driving circuit which cuts off a servo output 14 so as to diagnose the actuator 12 and drives the actuator 12 with the driving output 15 of an open loop and a dummy feedback circuit which cuts off the sensor output 16 so as to diagnose the servo driver 11 and simulates the sensor output into the lines of the servo output 14 and sensor output 16 connecting the actuator 12 and servo driver 11.