PURPOSE: To display high control ability even by a control system which has a long dead time by easily determining a control period from the sampling period of an analytic meter and securing proper operation based upon a proportional item.

CONSTITUTION: The controller which performs operation by using a control constant predetermined on the basis of deviation as a difference between a command and a controlled variable and calculates a manipulated variable to be applied to a process is provided with a control period arithmetic means 32 which determines the control period from the varying sampling period of a controlled variable measuring instrument such as the analytical meter, an integral item arithmetic means 33 which performs the operation of an integral item by using the control constant predetermined on the basis of the deviation of the controlled variable from the command and the control period found by a control period arithmetic means 32 and finds an integral item arithmetic output, a proportional item arithmetic means 34 which performs proportional item arithmetic by using the deviation of the controlled variable, and an adding means 35 which finds the manipulated variable by adding both the arithmetic outputs of those arithmetic means 33 and 34.