To accurately control a calibrating pressure level to pressure at using time by determining a reference mechanical parameter to show torque applied to an input shaft, putting an electronic control module in a calibrating mode condition, and selectively controlling a level of an electric current to a related solenoid actuating valve.

When calibration of a clutch is started, an electric current rises to a high level for a prescribed time, and opens a solenoid control valve 30 of the clutch to be calibrated, and is reduced to a minimum hold level, and is held on a hold level for a prescribed time to allow the clutch to be filled, and is substantially reduced to a 0 level in the next place, and is held for a prescribed time, and allows the clutch to be sufficiently discharged. A sequence of rising an electric current command, of reducing it to a hold level higher than the last time hold level and of substantially reducing an electric current to 0, is repeated until the hold level reaches time when the clutch is completely engaged.