PURPOSE: To improve watertightness and to prevent the leakage of hot water.

CONSTITUTION: A female screw part 25 consisting of tooth-shaped threads is formed on the inner periphery of the inner part of a tank body 16. An annular taper surface 26 is formed below the female screw part 25 so that the nearer to its upper end, its diameter is more expanded. A male screw part 28 consisting of tooth-shaped threads is formed on the outer periphery of a cover body 17. An annular circular groove 29 is formed below the male screw part 28. A cover body 17 is screwed into the tank body 16 with an O-ring 30 being arranged in the annular groove 29. At this time, with the screwing of the cover body 17, the O-ring 30 is pressurized to the taper surface 26 of the tank body 16 and deformed to improve the watertightness in the screwed part.