To prevent a plug cap of a protection cap for a hot water plug from unexpectedly separating from a plug cylinder.

Thick portions 11a, 12a projecting inward along the whole peripheral area are formed on inner peripheral faces of the plug caps 11, 12, only the thick portions 11a, 12a of the inner peripheral faces of the plug caps 11, 12 can be closely kept into contact with air-tight members 27, 28 when the plug caps 11, 12 are externally fitted to the plug cylinders 21, 22, and the thick portions 11a, 12a are closely kept into contact with the air-tight member 28 at the termination of mounting of the plug caps 11, 12 to the plug cylinders 21, 22, thus the plug caps 11, 12 can be externally fitted to the plug cylinders 21, 22 in an outer periphery air-tight state.

COPYRIGHT: (C)2008,JPO&INPIT