A power device 1 has a speed reducer 3 and a spring member 4 provided in a power transmission system between an actuator 2 and a driven element 5, an operation target of the actuator 2 is determined according to a linear combination of values that are obtained by passing through low-pass filters, a deviation Δω12 between an estimated value ω1—e of a velocity of an input unit 4a of the spring member 4 and an estimated value ω2—e of a velocity of the driven element 5, and a deviation Δθdef between a value θdef—s of a deviation between the input unit 4a of the spring member 4 and the driven element 5, and a target value θdef_cmd of the displacement difference corresponding to a target driving force τref of the driven element 5.