A casing (31) is provided around a power transmission shaft (3a) interposed between a crankshaft (2a) of an engine (2) and a power transmission device (3). A dynamo apparatus (10) and a cooling fan (23) cooling the dynamo apparatus are constructed in the casing (31) around the power transmission shaft (3a) and are driven by the power transmission shaft (3a). The casing (31) is cylindrical and comprises a connection surface at a side of the engine (2), a connection surface at a side of the power transmission device (3) and an outer peripheral surface between the connection surfaces. Intake holes (3d) are formed in the connection surface (3e) of the casing (31) at the side of the power transmission device and the intake hole is formed at an inner peripheral side of a blade of the fan, and are arranged on a circle centering on the axial direction of the connection surface (3e) at an inner peripheral side of blades (23a) of the fan (23). An exhaust hole (21b) is formed in a portion (21a) of the casing (31) at the side of the engine.