PURPOSE: To obtain the dryness degree of generated steam and to make it possible to correctly control the supply water quantity by amalgamating a cooling water path with a steam bypass line and mounting a temperature detecting element at a suitable position on the side of a joining point outlet.

CONSTITUTION: Water to be supplied to a heat exchanger 2 of a water heater is heated by a burner 1 and turned out to be steam. Steam passes through a steam passage 4 and is injected into a bathtub 7 through a nozzle 5. On the other hand, steam which has flowed into a bypass line 9 is cooled by cooling water which joins the steam from a cooling water path 8, and is condensed to hot water of a predetermined temperature. The flow quantity of steam flowing through the bypass path 9 is substantially proportional to the dryness degree thereof, and the flow quantity of steam and the dryness of steam are determined from the temperature of hot water. As a result, a controller 13 controls the opening degree of a water proportion valve 15 to obtain the flow quantity of steam which is in a proportional relation with the dryness of generated steam from the temperature detected a temperature detecting element 11 and to control the flow quantity of generated steam.