PURPOSE: To achieve higher connection efficiency eliminating idle boiling and burns by a method wherein a bathtub is arranged to be a double hollow structure of inner and outer tanks and a hollow part thereof is filled with an unfrozen liquid to heat it from the bottom surface of the outer tank.

CONSTITUTION: A bathtub 1 is arranged to be a double hollow structure of an enamel-finished outer tank 2 and an inner tank 3 formed by a stainless steel and a hollow part is filled with an antifreeze 4. Water is injected into the inner tank 3 to store. The bathtub 1 is heated by a flame from a burner 18 from the bottom surface of the outer tank 2 thereof 1. The antifreeze 4 is heated and the water in the inner tank 3 is heated with the transfer of the heat. As a result, there is no fear of idle boiling even if no water is in the inner tank 3 as the antifreeze 4 is always heated. Under such a condition, while a towel, mat or the like is placed in the inner tank 3, the bathtub can serve as a sample sauna bath. The control with a microcomputer 13 allows the obtaining and maintaining of moderate temperature condition of hot water as desired and heating of the water in the inner tank from the perimeter achieves higher convection efficiency.