A plant control system comprising: a process computer configured to manage a plant; a programmable logic controller interposed between said process computer and equipment of the plant and configured to execute an operation in a fixed period; and an information processor composed of a personal computer which is inferior in reliability in terms of punctuality as compared with said programmable logic controller and configured to execute arithmetic processing based on arithmetic data received from said programmable logic controller and transmit arithmetic result data thereof to said programmable logic controller, wherein said programmable logic controller comprises a transmission processing unit configured to extract the arithmetic data from data inputted from the equipment of the plant into said programmable logic controller and transmit the arithmetic data to said information processor; characterized in that the programmable logic controller comprises a unit configured to add a counter value, which is counted up in every transmission of the arithmetic data, to the arithmetic data to be transmitted by the transmission processing unit, wherein said information processor comprises a unit configured to add the same counter value as the counter value, which has been added to the arithmetic data being a base of the arithmetic result data, to the arithmetic result data to be transmitted to said programmable logic controller, and wherein said programmable logic controller comprises a consistency check unit configured to examine whether or not the arithmetic result data corresponding to the transmitted arithmetic data is received within a predetermined time period, based on the counter value added to the arithmetic result data received from said information processor and the counter value added to the arithmetic data which has been transmitted by the transmission processing unit for evaluating validity of the arithmetic result data received from said information processor. a process computer configured to manage a plant; a programmable logic controller interposed between said process computer and equipment of the plant and configured to execute an operation in a fixed period; and an information processor composed of a personal computer which is inferior in reliability in terms of punctuality as compared with said programmable logic controller and configured to execute arithmetic processing based on arithmetic data received from said programmable logic controller and transmit arithmetic result data thereof to said programmable logic controller, wherein said programmable logic controller comprises a transmission processing unit configured to extract the arithmetic data from data inputted from the equipment of the plant into said programmable logic controller and transmit the arithmetic data to said information processor; characterized in that the programmable logic controller comprises a unit configured to add a counter value, which is counted up in every transmission of the arithmetic data, to the arithmetic data to be transmitted by the transmission processing unit, wherein said information processor comprises a unit configured to add the same counter value as the counter value, which has been added to the arithmetic data being a base of the arithmetic result data, to the arithmetic result data to be transmitted to said programmable logic controller, and wherein said programmable logic controller comprises a consistency check unit configured to examine whether or not the arithmetic result data corresponding to the transmitted arithmetic data is received within a predetermined time period, based on the counter value added to the arithmetic result data received from said information processor and the counter value added to the arithmetic data which has been transmitted by the transmission processing unit for evaluating validity of the arithmetic result data received from said information processor.