Abstract: An air-cooled internal combustion engine and a structure of its air cleaner, a major object of which is to control noises emitted from various parts of engine and to minimize the overall size and dimensions of the engine. A cam shaft and a balancing shaft are arranged on an air cleaner side relative to a crank-shaft, a starting motor is arranged on an exhaust muffler side relative to the crank-shaft, shaft portions of the balancing shaft and the starting motor are located at a position lower than the crank shaft, and the exhaust muffler is arranged vertically above the starting motor, so that a center of gravity of the entire engine is lowered and engine noise can be controlled. Further, noises emitted from various parts of engine are controlled by positioning and shaping of an exhaust pipe, a shape of a cylinder head cover, and an arrangement and position of a fuel tank.