PURPOSE: To secure smooth connecting characteristics to prevent a shock at starting by keeping the oil pressure of a hydraulic cylinder at a low value, which is obtained by learning and does not cause any shock, until just before a full stroke in engaging a hydraulic clutch.

CONSTITUTION: A transmission 15 is set at its neutral position, and a clutch 30 is made to carry out a stroke in a connecting direction through a computer 21. Since oil pressure therefore rises sharply at the end of the stroke, a time required until then is obtained by learning. Voltage applied to a pressure control valve 32 is then slowly decreased, and then oil pressure is read in an oil pressure sensor 31, and a current corresponding to the oil pressure rising where the oil pressure can hardly rise, and a shock is not caused is obtained by the learning. The above learning time and current are stored in the computer 21, and after a time obtained by deducting a fixed time from a time required for the stroke elapses, the current applied to the control valve 32 is reduced to the above learning current. The coil current of the control valve 32 is increased after then, and can thereby be kept low until the end of the stroke.