Provided is a method of detecting and preventing soft errors without performing multiplexing. As a result, it is possible to improve reliability of a device while suppressing an increase in mounting cost and reduction in operation speed accompanied with it. A diagnosis system includes an initial parameter generation unit which generates a plurality of initial parameters predicted on the basis of an external input; an operation unit which has a plurality of operators operating optimal solutions to the initial parameters by using an evaluation function describing a control object; and a diagnosis unit which diagnoses the operation unit on the basis of an output of the operation unit. When an optimal solution candidate giving an evaluation value deviating from a value taken by each evaluation value by a constant threshold or more among evaluation values corresponding to the optimal solutions to the initial parameters is found, the diagnosis unit diagnoses an error of the operation unit.