PURPOSE: To surely prevent falling-off of a plug by arranging a fitting part of the plug for closing an opening side end of an oil hole opposite to a projection provided on or integrated with an inner wall of a support body in such a manner that a clearance between the plug fitting part and the inner wall/ projection is made shorter than the axial length of the plug.

CONSTITUTION: A plug 18 is beforehand fitted to an upper end of an oil hole 16a equipped in a pump housing 16 and an upper end opening of the oil hole 16a is closed by the plug 18. A fitting part 16d of the plug 18 is adjacent to respective inner walls 13b, 19a of a transmission case 13 and an extension housing 19, and a clearance S formed between the fitting part 16d and the inner walls 13b, 19a is shorter than the axial length L of the plug 18. With this constitution, even when the plug 18 is loosened and falling-off of the fitting part 16d, a tip of the plug 18 abuts against inner walls 13b, 19a of the transmission case 13 and the extension housing 19 so as to prevent complete falling-off of the plug 18.