PURPOSE: To improve durability and maintainability of a drain plug by hanging down a hanging down piece capable of swinging from a plug cover opening/ closing a drain port by up and down motion, and letting the piece cooperate with a crankpin converting rotational motion to up and down motion.

CONSTITUTION: A cylinder 47 internally mounted with a piston 50 is threadedly engaged with the plug cover 41 of a drain plug 3 through a nut, and in addition a hanging down piece 48 is hung down from the lower end of the cylinder 47 capably of swinging. A rotary shaft 62 is integratedly formed with a crank projecting piece 63 outward projecting on the extreme end side, a crank 64 is projected on the extreme end of the crank projecting piece 63, and the projecting piece 63 is insertedly fitted to a universal joint 4. Next an engaging groove is formed on the hanging down piece 48, and the crankpin 64 of the rotary shaft 62 is engaged with the groove. By rotating the universal joint 4 through an operation part, torque is transmitted to the rotary shaft 62 through a cylinder body 60, the crank projecting piece 63 and the crankpin 64 are rotationally moved, rotational motion is converted to up and down motion, and hence a drain port H is opened/closed.