An actuator fault detection device that detects a fault of an actuator driven according to a manipulation variable MV output from a controlling device, including: a position acquiring portion that acquires a value of a motor position MP from a detector that detects a position of an actuator corresponding to a manipulation variable MV; a follow-up difference calculating portion that calculates a follow-up difference DM=MV−MP; and an allowable range determining portion that determines whether or not high speed operation or low speed operation of the actuator departs from an allowable range based on a manipulation variable change rate ΔMV, an actuator position change rate ΔMP, and the follow-up difference DM.