To provide a bath hot water supply heating system capable of adding a heating function by post-mounting a heating device to an existing bath hot water supply heater.

A heating circuit, which comprises a heater 1 set up in a bathroom and a water storage tank 201 which are connected to each other in series, is connected to a bathtub 4 in parallel and is arranged to form a delectable distribution passage. When this heating circuit is selected, a water level sensor 315, which detects the water level of the bathtub 4 is arranged to be replaced with a water level sensor 202 which detects the water level of the water storage tank 201 so as to make effective the output of the water level sensor 201 so that the water storage tank 201 may serve as a dummy bathtub. After hot water is filled up in the water storage tank 201 by a hot water fill-up action, high temperature hot water is distributed by additional heating by way of the heater 1, thereby performing heating operation.