To solve the problem that when four bolts are tightened at positions at unequal intervals, the bolt-tightening force in the outside-peripheral flange on a long interval side is not so strong as that in other parts, and it has a chance of causing deformation to degrade the sealing property.

In the case when four bolts 72 are tightened at unequal intervals, rigid members 73 are contact-placed on the upper face of the outside peripheral flange 62a as a bridge between the bolts placed at long intervals. The four bolts 72 are tightened in the state in which the rigid members 73 are placed on the outside peripheral flange 62a of an upper cover 62. Thus, the flange 62a on the side of long bolt-interval is also pressed to an upper case by the tightening forces of respective bolts 72 transmitted through the rigid members 73, and deformation of the flange 62a is restrained. Consequently, sealability between the upper case and the cover 62 is ensured over a long period of time.