To prevent generation of steam hammer sound, by forming a pipeline to a drain discharge port from a steam supply port at an oblique passage descending toward the discharge port, and forming a pipeline of a heating tube substantially in a V shape, thereby preventing retention of the drain in the tube.

A pipeline of a heating tube 2 is formed at an oblique passage descending from a steam supply port 3 toward a drain discharge port 4. A shape of the passage is, for example, formed by bending substantially in a V shape, and the tube 2 of the V shape is arranged horizontally. Thus, the passage descending from the port 3 toward the port 4 is formed. Accordingly, when steam is supplied into the tube 2 to heat water or hot water in a tank body 1 by heat exchanging it by condensing, the generated drain in the tube 2 is not stayed in the tube 2.