PURPOSE: To enhance the moving responsiveness of a cylindrical gear from a foremost position to an intermediate position by restraining the contact pressure between two gear mechanism parts from increasing.

CONSTITUTION: A cylindrical gear 28 meshed between a timing pulley 21 and a cam shaft 22 is controlled so as to be moved axially in three stages by means of a moving member 34. Further, during shifting from an engine idling operation to a low speed range, a controller 54 controls solenoid valves 50, 51 so as to feed hydraulic pressure into a first pressure receiving chamber 35, and simultaneously, hydraulic pressure is temporarily fed into a second pressure receiving chamber 36. Accordingly, both front and rear gear mechanism parts 29, 30 are simultaneously moved to an intermediate position under one and the same pressure.