To prevent cooling convection due to outside air when a separate type bath boiler is extinguished and cooling convection due to outside air from occurring and shorten re-heating time at the time of re-bathing by providing a check valve in a lower side pipe.

A check valve 1, comprising a main body having a tapered valve seat and a ball float valve element 2 having a weight adjusted so as to set it afloat by hot water flow at the time of water heating and be seated on the valve seat when a separate bath boiler is extinguished, is installed in an inlet of a lower side pipe 6 connecting a bathtub 5 and the separate bath boiler so as to avoid cooling convection. A lower end of the check valve 1 is provided with a nozzle 3 having a length approaching a bottom of the bathtub 5 so that a hot water layer at a bottom part of the bathtub 5 where low temperature stagnates is sucked when water is heated, and an upper pipe 7 connecting the bathtub 5 and the separate bath boiler is provided with a nozzle 4 having a height than an upper end of the pipe 7 so as to avoid cooling convection.