To facilitate assembly of an auxiliary machine drive shaft by arranging an oil level inspection window on an extended line of an axis of the auxiliary machine drive shaft and utilizing an aperture for the inspection window for assembly of the auxiliary machine shaft.

The oil level inspection window 9 is provided on an aperture 10B2 of a divided lower case side wall 10B1 of a crank case, and the aperture 10B2 is positioned on the extended line 0-0 of the axis of the auxiliary machine drive shaft 4. Assembly of the auxiliary machine drive shaft 4, such as attachment of a sprocket 4A to a drive shaft end 4a in assembly of the shaft 4, is performed before fitting of the oil inspection window 9 to the aperture 10B2, and can be easily performed with the usage of the aperture 10B2.