To select an optimal control system suitable for safety required by a plant or the like by quantitatively evaluating the reliability of a controller.

Output signals 5a' and 5b' and 5a" and 5b" of arithmetic circuits 6 and 7 for self-diagnosis having the same input signal system and logical arithmetic operation as an arithmetic circuit 3 for control integrated in a controller 1 are compared with output signals 5a and 5b of the arithmetic circuit 3 for control, and a function loss time ti during the generation of any difference between those signals is integrated. On the other hand, a total operating time T since a power source 16 is turned on for the controller 1 is integrated, and a total function loss time X ti is divided by the total operating time T so that function loss probability P can be calculated. Thus, the reliability of the controller 1 can be quantitatively evaluated based on the function loss probability P.