[en] The present invention provides a control device for a nuclear power plant by using a computer having a simple constitution and a high reliability. That is, (1) Control devices are dispersed for averaging load (software amount) on each of the control devices. (2) Each of the control devices diagnoses only the control devices at an inlet and an outlet, to facilitate identification of abnormal portions. (3) Each of the control devices comprises a logic in the computer which properly copes with abnormality concerning the control of its own if it should be caused in the plant, to perform control by an appropriate logic based on an inputted process amount. A function of a higher rank computer is enlarged in an existent control device having a stepped constitution to cause troubles in view of design, maintenance and reliability, but such troubles are solved in the present invention since the device of the present invention has such functions described above. (I.S.)