PURPOSE: To perform the process control with high responsiveness by fetching successively the present value of the process input data, also fetching the sampled and stored data against the present value of the process input data, and then calculating the changing rate, etc., of the process input data in an input cycle shorter than the sampling cycle.

CONSTITUTION: The sampling cycle Ts of the process input data is set at one second together with the sampling number Ns set at 2 senconds, and the control cycle Ta of a process controller set at 0.2 seconds, respectively. Therefore, the storage capacity N of a memory 5' is set at 6. Under such conditions, the numerical value 0-5 are set at an address index Ii corresponding to an address Ai of the memory 5'. Then, the process input data are stored in the memory 5' at each control cycle T, and the data stored in the memory 5' earlier than the presently stored data by a single sampling cycle Ts, i.e., one second are taken out as the precedent value. Then, the changing rate of the process input data is calculated by an arithmetic processing circuit 7 based on the sampling data.