To assemble an oil pump separately from a crankcase so as to improve assembling efficiency and maintainability by fitting a holder to which a crankshaft is journalled to a crankcase, and forming a pump housing by the holder and a cover fastened thereto.

A right holder 34 rotatably supporting a crankshaft 20 is fitted to a surface 108 to be fitted of a case half part 25 in a crankcase 26. A pump housing 90 for an oil pump 82 is formed by the right holder 34 and a cover 91 fastened to the holder. By such arrangement, the oil pump 82 can be assembled on the right holder 34 side fitted to the crankcase 26, and the right holder 34 fitted with the oil pump 82 can be fitted to the crankcase 26, so that assembling efficiency and maintainability of the oil pump 82 can be improved.