passage 20 a of oil pump 20 to first fluid passage 34 and connect drain passage 22 to second fluid passage 35 when now an ignition switch not shown of the engine is turned on oil pump 20 becomes powered by the cranking of the engine upon this as is seen from fig7 a certain amount of hydraulic fluid is led to pin engaging chamber 31 through first fluid passage 34 and also to first engaging recess 24 through one advancing chamber 12 and connecting passage 42 with this fluid supply the engagement between first engaging pin 26 and first engaging recess 24 becomes much tightly made while the engagement between second engaging pin 27 and second engaging recess 25 is kept without change in engaging force when after completion of the cranking the engine takes a transit condition just before starting its idling operation hydraulic control mechanism 28 takes such a condition as depicted by fig8 that is upon such condition an instruction signal is fed from the control unit to second electromagnetic switch 36 causing the same to take another condition wherein the spool takes the other position to connect drain passage 22 to first fluid passage 34 and connect outlet passage 20 a of oil pump 20 to second fluid passage 35 with this pin engaging chamber 31 is subjected to a pressure decrease.