rotor 30 here the timing sprocket 51 is constructed as is well known in the art to transmit the rotating power to the clockwise direction of fig3 from a crank pulley 53 through a timing chain 54 the cam shaft 10 is equipped with the well known cam 11 for opening and closing an exhaust valve not shown and is provided therein with an advance passage 12 and a delay passage 13 which are extended in the axial direction of the cam shaft 10 the advance passage 12 is connected to a connection port 91a of a housing 91 of a change over valve 90 via an annular passage and a connection passage 71 which is located within the cylinder head 70 on the other hand the delay passage 13 is connected to a connection port 91b of the housing 91 of the change over valve 90 via an annular passage and a connection passage 72 which is located within the cylinder head 70 here there is a bolt 14 at one end of the delay passage 13 so as to close the end of the passage 13 the change over valve 90 as shown in fig1 and 2 is enabled to move a spool 92 with a moveable core 94 leftward of fig1 against the action of a coil spring 93 by energizing a solenoid.