To facilitate work on securing a coupling cover, by forming, at the front of the coupling cover, insertion portions temporarily secured to concave steps of grommets on the engine-side and, at the rear thereof, a bolt securing portion to be secured to a supporting member of the impeller-side from above with a bolt.

On the rear surface of a crankcase 41, brackets 41a, 41b which have grommet securing holes, respectively, are integrally formed so as to correspond to insertion portions of flanges 45e, 45f of a coupling cover 45. Further, on the upper portion of a bearing housing 42, a support 42b is integrally formed so as to correspond to a bolt securing portion 45i of the coupling member 45. In assembly, first, the respective insertion portions of the flanges 45e, 45f at the front are inserted into small-diameter portions of the respective grommets secured to the brackets 41a, 41b of the crankcase 41, and temporarily secured. Then, the bolt securing portion 45i of the coupling cover 45 is settled onto the support of the bearing housing 42 from above, and secured with one bolt.