PURPOSE: To reduce the number of parts and noise, compared with the case of conventional gear drive, by driving a governor mechanism for rotating a crank shaft connected to a cam shaft through a timing belt at a fixed speed by the use of the timing belt.

CONSTITUTION: An OHC type engine 1 has a crank shaft 5 and a cam shaft 14 interlocked and connected to each other by a timing belt 17 through respective pulleys. In this case, a governor mechanism 54 for rotating the crank shaft 5 at a fixed speed has an essential part formed by rotatably supporting a governor holder 56 driven by the timing belt 17 by a fixed shaft 55 built in a cylinder block 2. A governor weight 57 opened according to the engine rotation is received in the governor Holder 56. The movement of the governor weight 57 is transmitted to a throttle valve in an evaporator not shown through a governor slider 58 and governor arm 59 made into contact with the arm 57a.