PURPOSE: To permit stable control without generating hunting in one PID constant in the set temperature range of all of materials to be heated by a method wherein an operating output correcting value is related proportionally to a measured value.

CONSTITUTION: A PID controller 5 switches over a switch 10 to the terminal C1 side, and operates an operating output value MV by a PID operating unit 7 based on the temperature of materials to be heated after heat exchange, which is detected by a temperature sensor 6, or a measured value PV, the preset value SV of the temperature of the materials to be heated and a PID constant, then, operates the relational formula of the value MV and the value PV through a function operating unit 8. Next, the PID controller 5 switches the switch 10 to a terminal C2 side and corrects the operating output value MV to an operating output correcting value MV', proportional to the measured value PV, through an operating output correction operating unit 9, then, inputs it into a driving unit 4. In this case, there is a proportional relation between the operating output correcting value MV' and the measured value PV whereby the driving unit 4 is controlled and the set pressure of the reducing valve 3 is changed based on the operating output correcting value MV'.