To prevent water from staying in a drum by mounting drainage holes in the margin of the drum.

The drainage hole 4 is mounted at a position lower than an oil supply opening 3 of the drum 1 on the side surface of the margin 2 of the drum. A hollow tube is inserted into the hole and then a pressure is imparted thereto so that both sides of the tube opening is crushed outward to make an eyelet-like reinforcement tube 5 around the hole or a part 6 of the margin of the drum is arranged to lower than the position of the oil supply opening. The drainage hole of the drum is not mounted at one position, but a plurality of drainage holes are mounted at a plurality of positions. When one drainage hole is clogged, water can be drained through the other holes, so that the drainage effect is raised.