PURPOSE: To shorten the time to be required up to the arrival of a controlled system at an objective set point by preparing three elements for detecting a parameter to be controlled, adding respective detection values at the time of an initial state, and in a steady state, controlling the controlled system by using an intermediate detection value or maximum detection value.

CONSTITUTION: Detection values from three or more detection elements 1-1 to 1-z are applied to an arithmetic circuit 2. The circuit 2 searches an intermediate detection value among these detection values, and when the searched value is lower than a previously set threshold Th, applies an output signal obtained by adding respective detection values to the system 3 to be controlled soon after the controlled system 3 starts its operation. Since a large control signal is applied to the system 3 by adding the detection values of respective detection elements, an input signal level is increased as compared with the case of one detection element and operation in an initial state can be accelerated in accordance with the increment of the input signal level. When a parameter to be controlled is temperature, the temperature is suddenly raised.