PURPOSE: To effectively heat a material dipped in a hot water tank with a small quantity of water and a thermal energy by inputting an element to radiate a far infrared radiation when heated from a surface into the hot water tank.

CONSTITUTION: A hot water 2 made into a prescribed temperature is poured into a bathtub 1, and an element 3 to have a specific gravity smaller than the hot water 2 and to radiate the far infrared radiation when heated from the surface is inputted to the hot water 2. The element 3 floats on a hot water surface and radiates the far infrared radiation by being heated with the hot water 2. The element 3 to radiate the far infrared radiation is made after the material to radiate the far infrared radiation by the heating is applied or impregnated onto the surface of a lump to be made of a ceramic, a metal, a plastic, a wood, etc., and to have the specific gravity of '1' or below.