A construction is provided in which a drain hole 4 provided with internal threads 3 in a Mg alloy oil pan 1 is closed with a screw-type plug body 5 constituted by a metal other than Mg and Mg alloy. The plug body 5 has a threaded shank 6 screwed into the drain hole 4 and an enlarged end 7 having a diameter larger than that of the threaded shank 6. A corrosion preventing plate 14 fitted on the threaded shank 6 via a threaded shank passing hole 15 is disposed between a washer 11 of the enlarged end 7 and a peripheral portion 13 of an opening of the drain hole 4 which corresponds to the washer 11 so that there is provided an electrical insulation between the washer 11 and the peripheral portion 13 of the opening. The corrosion preventing plate 14 has a plurality of annular sealing projections on both sides thereof which are arranged in a concentric fashion about the threaded shank passing hole 15 so as to bite into the peripheral portion 13 of the opening and the washer 11.