A plant control system includes a process computer (1) managing a plant, a PLC (2) interposed between the process computer (1) and equipment (3) of the plant, and a general-purpose PC (7) executing arithmetic processing based on arithmetic data received from the PLC (2) and transmitting arithmetic result data thereof to the PLC (2). The functions are divided such that sequential control logic is executed by the PLC (2) and complicated arithmetic processing is executed by the general-purpose PC (7), and the arithmetic result data outputted from the general-purpose PC (7) always passes through the PLC (2) so that consistency of the arithmetic result data is checked in the PLC (2). This makes it possible to apply a general-purpose personal computer (general-purpose PC) to the plant control system so as to divide the functions between the programmable logic controller (PLC) and the general-purpose PC and to compensate the reliability of the general-purpose PC in an industrial application, for example, in terms of punctuality.