PURPOSE: To perform constantly excellent control even when a water feed amount is changed by a method wherein in drive of a gas proportional valve by means of a microcomputer control part, a current water feed amount is detected, and according to the water feed amount, and by changing the change parts of a proportional gain and an integral time, PID control is performed.

CONSTITUTION: A water feed amount is detected by a water amount sensor 7 disposed in a water feed pipe, and a detecting value thereof is inputted to a microcomputer control part 5. Simultaneously, based on a detecting values, KP, T1, and TD of a formula I are formed as in a formula of KP=A1×FW+A2, T1=B1×1/FW+B2, TD=C1×FW+C2, FW = a detecting water amount (l/min), wherein A1, A2, B1, B2, C1, and C2 are a constant. In which case, even in the case of a minimum flow rate, KP, T1, TD are not reduced to a value similar to zero, and proper values A2, B2, and C2 at a minimum flow rate are maintained for control. When a flow rate exceeds the above values, A1×FW, B1×1/FW, and C1×FW, respectively, are added to the above proper values, and an excellent feed hot water temperature can be provided at any flow rate.