To conduct water drainage work immediately after the stop of a warmer.

A communication pipe 30 whose one end is connected to the water feed/drainage pipe 26 of a water storage tank 7 is arranged along the lower side of the lower end of the water feed/drainage pipe 28 of a stream tank 8. The lower end of the water feed/drainage pipe 28 of the steam tank 8 is connected to the communication pipe 30 through a through-hole 31 throttled into a small diameter. The other end of the communication pipe 30 is connected to a drain pipe 34 with a drain valve 35 interveniently provided. When the drain valve 35 is opened, the water remaining in the water storage tank 7 and the high temperature hot water remaining in the steam 8 flow into the communication pipe 30 in confluence from the respective water feed/drainage pipes 26, 28, and are discharged through the drain pipe 34. As the water feed/ drainage pipe 28 of the steam tank 8 is connected to the communication pipe 30 in the state of being throttled, a small quantity of high temperature hot water is joined to a large quantity of water and mixed in the communication pipe 30, and as a result, water drainage is conducted in the state of temperature being lowered moderately.