PURPOSE: To prevent sudden engagement of a clutch, and to reduce shock through securing a half-engaged clutch state by forming many drain holes on a spool which withdraws with inlet of hydraulic oil, and by opening and closing the respective drain holes in sequence with the reciprocating slide motion of the spool.

CONSTITUTION: A spool 8 is fitted slidably inside a valve body 9 inserted into a valve case 13. A piston 5, which slides in a back chamber E, is fitted on a guide rod part 8b of the spool 8 and also fitted slidably inside the valve body 9. Furthermore, pressure oil is supplied from an oil passage 12 of a case cover 1 to an oil chamber F, and after that, is introduced into the back chamber E from an orifice hole 3 in an opening 8a of the spool 8 through a drain opening 6 and grooved oil passages G. At this time, many drain oil passages A-D are formed in the opening 8a. The respective drain oil passages A-D are not communicated with the grooved oil passage G and are opened at the outer periphery of the spool 8. So hydraulic oil is returned to the drain line only when the drain oil passage A-D are communicated with the drain opening 6.