the front thrust surface 63 the provision of these oil passages 75 and 78 sufficiently lubricates the front thrust surface 63 the intake camshaft 3 is further formed with a radial oil passage 79 extending from the axial oil passage 74 and opening to the outer surface of the rear journal 62b the radial oil passage 79 is axially located in a position closer to the front end of the rear journal 62b than to the rear end of the rear journal 62b a part of the oil passing in the axial oil passage 74 is delivered to the radial bearing means 46 through the radial oil passage 79 the radial oil passage 79 located closer to the front end of the rear journal 62b allows the major part of lubrication oil passed throughout the radial oil passage 79 to flow towards the rear thrust surface 65 so as to lubricate sufficiently the rear thrust surface 65 after the lubrication of the rear thrust surface 65 the oil is returned through a groove 81 formed in the thrust collar 64 similarly a branch oil passage 83 branching off from the exhaust camshaft oil passage 82 extends to a radial bearing 49 so as to lubricate the radial bearing portion 49 a part of the oil passed through the branch oil passage 83 enters into an undercut groove 84 formed inside the internal thread bore 70 and then is returned through a return oil passage 85 so as to lubricate the front thrust metal insert 67 and the friction gear 61 the exhaust camshaft 4 is further formed with a radial oil passage 86 extending from.