PURPOSE: To reduce a clutch shock by draining working oil through a back-pressure path for a working oil pressure regulation valve having a loosely fitting-in valve at the transient position of a speed switching valve when it is switched with a forward/ reverse switching valve in the forward position state.

CONSTITUTION: When a speed change valve 4 is switched, under the condition that the forward/reverse switching valve 3 is in forward position 3b, working oil is drained through a back pressure path 21 for a working oil regulation valve 5 having a loosely fitting-in valve, in its transient position 4c, while a pressure adjusting path 20 being opened and working oil pressure being dropped to the first set pressure of oil paths 9, 11. And after the speed switching valve 4 is switched, a forward clutch 2a is placed in a connected state until the oil pressure rises to the second set pressure, thus reducing the shock caused by clutch engagement when the speed change valve 4 is switched. Also, when the forward/reverse switching valve 3 is in its reverse position 3a, the working oil in the oil path 9 flows through the pressure regulation path 20 of the pressure regulation valve 5 and is supplied to a reverse clutch 1 after being gradually contracted by a loosely fitting-in valve 22, thus the shock is reduced.