To reduce the weight of a cam shaft and lessen the driving force thereof by setting the width of a sliding contact surface in a base circle region smaller than the width of a sliding contact surface in a nose circle region in a cam lobe part of a cam shaft for driving a suction and exhaust valve of an internal combustion engine.

A cam lobe part 10 of a cam shaft 1 is formed of a base circle part S with a radius R3 and a cam nose part N with a radius R4, and provided with a sliding contact surface 11, a side surface 12 and a chamfered part 13. In this case, an N1 portion of the sliding contact surface 11 has the width L1 set to the maximum for receiving large force. In a region extending from B to A of a N2 portion and a region extending from C to D of an N3 portion, the width is made smaller, and the width L2 of the base circle part S is set to the minimum. According to the width of each region in the sliding surface 11, the chamfered part 13 is disposed to form a cam lobe 10. Thus, the weight and inertia moment of the cam shaft 1 are reduced to lessen the driving force.