An amplitude control unit (40) detects the amplitude of a movable element (12) that reciprocates relative to a stator (11), detects fluctuations in a load over time according to detected fluctuations over time in the amplitude, and detects an abnormality on the basis of the detected fluctuations over time in the load. The amplitude control unit (40) outputs the detected amplitudes to a control output unit (50). The control output unit (50) controls a drive current (Id) for reciprocating the movable element (12) on the basis of the amplitude information supplied from the amplitude control unit (40).
