A crankshaft phase adjustment structure for performing work via a single window. A clutch and ACG are arranged on both right and left sides of a crankcase. On the side of the clutch of the crankshaft, a primary drive gear engaged with a primary driven gear of the clutch is fixed by a bolt. A window for performing work is open to a case cover in the vicinity of the bolt and is covered with a cap. The center line of the window for performing work is set off the axis of the bolt, the window for performing work is located in a position in which a head of the bolt and an alignment mark provided to the outside periphery of the side of the primary drive gear are simultaneously viewed. At the edge of the window for performing work a second alignment mark is provided.