In the case of a cam shaft drive structure in which a cogged belt is passed around a pair of cam shaft pulleys for two cylinder banks of an OHC V-type engine and a crank shaft pulley, it has been widely believed that the tension of the cogged belt resulting from the longitudinal vibration of the cogged belt can be minimized by reducing the masses of the cam shaft pulleys to a least possible value. However, it was discovered by the inventors that it is not necessarily the case and on the contrary a favorable result can be obtained by selecting the moment of inertia of the cam shaft pulley on the tight side to be greater than the moment of inertia of the cam shaft pulley on the slack side. The reverse surface of the cogged belt can be conveniently utilized for driving accessory equipment and for engaging an idler pulley thereto.