A rotational balance adjustment structure for an engine includes; a damper pulley having an elastic member provided on the outer periphery of a hub and a damper mass provided on the outer periphery of the elastic member; and an engine assembly having an engine body and a plurality of parts mounted on the engine body, the plurality of parts including the damper pulley. The engine assembly is assembled by mounting a plurality of parts to the engine body, the plurality of parts including the damper pulley, and the rotational balance of the engine assembly is adjusted by attaching a balance piece to the damper mass.