PURPOSE: To achieve smooth inter-clutch switching by coupling an orifice and a one-way valve in parallel with a first clutch drain port thereby avoiding such condition where two clutches are blocked when they are switched hydraulically.

CONSTITUTION: An orifice 70 is arranged in a drain oil path coupled with a first clutch drain port 62 of a clutch control valve 50, and a one-way valve 71 for allowing oil flow only from the drain port 62 to a second solenoid valve 46 is arranged in parallel with the drain oil path. When a first clutch 20 is disengaged and a second clutch 5 is engaged, hydraulic pressure of the first clutch is discharged through the drain port 62 of the clutch control valve 50. Since the one-way valve 71 is closed upon turning ON of the second solenoid valve 46, hydraulic pressure of the first clutch is discharged only through the orifice 70 so as to loosen falling of clutch hydraulic pressure. Consequently, both clutches are not disengaged simultaneously, resulting in smooth switching of clutch.