PURPOSE: To make it possible to control the amount of supply fuel to a burner when starting fuel supply and prevent the generation of back-fire by providing a proportional valve near a solenoid valve and energizing the proportional valve simultaneously with the solenoid valve when the fuel supply is started.

CONSTITUTION: When the temperature of water is set by a temperature setting volume switch 52 and hot water supply is started, a control circuit 50 is actuated by a water- flow switch and a source solenoid valve 43 and ignitor 26 are actuated. When the ignition is detected, a main solenoid valve 44 is energized and at the same time a proportional valve receives electric current for slow ignition. At this time the fuel gas that passes through a gas pipe 42 goes through the main solenoid valve 44 and attempts to go through the proportional valve 45 as well. But because the proportional valve 45 has a slower displacement speed as compared with the main solenoid valve 44, the former can not finish its displacement at the same time as the latter even if both are energized at the same time. Further because the valve 45 is installed near the main solenoid valve 44, the pressure of the fuel gas is applied to the proportional valve 45 as soon as the main solenoid valve 44 is opened and it is capable of presenting opening corresponding to the flowing electric current and it is possible to control the supply volume of the fuel gas.