PURPOSE: To improve a control characteristic of a thrust control, by mounting a feedforward control circuit which compensates for reduction in phase delay gain by differentiating an instruction thrust by means of a time.

CONSTITUTION: A thrust F corresponds to a torque which a DC servo-motor 104 should output, and if a current flowing to a field system 105 of a motor is constant, the torque of the servo-motor 104 proportions an armature current, whereby the torque of the servo-motor 104 or a thrust F for driving a wave- making device can be detected by means of the armature current. The set thrust F is a current instruction value, and thereby, a thrust braking is performed by a current control system which compares it with an actual armature current. A feedforward control is then added, and in which case, the signal of the feedforward compensates for an answer delay and reduction in gain by the use of a differential signal of a thrust set value. This permits the improvement of a thrust control characteristic.