A decompression shaft (56) of a decompression device (50) includes an engagement pin (53) that is guided by a guide groove (51a) formed in a decompression weight (51), a decompression cam (54) that is provided on one cam surface of an intake valve cam (25c) and an exhaust valve cam (25b) so as to advance and retreat, and a connection portion (55) that connects the engagement pin (53) and the decompression cam (54). The decompression weight (51) is formed with a rotation restricting groove (51e) that restricts rotation of the decompression shaft (56) when a force acts in a direction in which the decompression cam (54) moves on the decompression shaft (56) from an advanced position to a retracted position when an engine (E) is stopped and that is continuous with the guide groove (51a).