PURPOSE: To eliminate the useless space by a method wherein a heat exchanger is fixed on the outer wall of the bathtub.

CONSTITUTION: The heat exchanger 1 is arranged within a space 3b, which is defined with the outer wall 3a of the bathtub 3 and a horizontal 13, self-contained with a spirally wound hot water circulating pipe 2 and provided with an outlet and an inlet for hot water at the vertical side surface part facing to the direction of the thickness of the heat exchanger 1. An incoming hot water pipe 11 and the hot water circulating pipe 2 are jointed at the inlet and an outgoing hot water pipe 12 and the hot water circulating pipe 2 are jointed at the outlet. The connectors 5 are fitted and fixed from the inside of the bathtub 3 at the side surface of the heat exchanger 1 by pinching the outer wall 3a of the bathtub 3 between packings 14 and the side surface of the heat exchanger 1.