PURPOSE: To increase the thermal efficiency of a bath boiler main body by utilizing heat in exhaust gas in maximum.

CONSTITUTION: The uppermost layer of a bath boiler main body 1 is communicated with the lower side of a bathtub 10 through a communicating pipe 2a and the lowermost layer of the bath boiler main body 1 is communicated with the upper side of the bathtub 10 through another communicating pipe 2b while at least one of the communicating pipes is provided with a line pump 7 so as to produce the flow of water from the uppermost layer in the bath boiler main body toward the lowermost layer of the same. Water in the lower side of the bathtub 10 is introduced into the bath boiler through the communicating pipe 2a while water, passed through a circulating passage in the bath boiler, is introduced to the upper side of the bathtub 10 through the communicating pipe 2b whereby the heat of the stream of exhaust gas can be recovered efficiently.