To avoid fluctuation of control due to adverse influence of an outer magnetic field and low temperature environment, reduce manufacturing costs by reducing the number of parts, reduce the whole size of the appliance and improve hot water discharge characteristics.

A heat exchanger 3 receiving supply of water as a heating object and a combustion burner 4 for heating the heat exchanger are provided on a combustion can body 2. A gas flow control valve 13 is provided in a gas supply pipe 10 for supplying fuel gas to the burner. A gas on-off valve 14 is provided on each of combustion nozzles 41. The gas flow control valve is a control valve having a piezoelectric element drive type driving part and capable of changing and regulating the valve opening by extension and contraction according to the magnitude of applied voltage. Each of the gas on-off valves in a control valve having a piezoelectric element drive type driving part and opened by application of a voltage. The supply flow rate of the fuel gas to the combustion burner side is changed and controlled by the magnitude of the applied voltage to the gas flow control valve and the change and regulation of combustion output by the combustion burner is controlled by the number of the gas on-off control valves to which the voltage is applied.