To highly accurately diagnose system abnormality in a system whose feedback is controlled by sliding mode control without being influenced by disturbance.

A linear term manipulated variable UL is calculated by adding a proportional manipulated variable UP obtained by multiplying an error variable by P gain c to a speed manipulated variable UN obtained by multiplying the differential value of a practical controlled variable by speed gain d, a switching function S is calculated by adding a value obtained by multiplying the error variable by an inclination γ to the differential value of the error variable and a nonlinear term manipulated variable UNL is calculated as a smoothed function -kS(S+δ) using the switching function S. When prescribed time has elapsed from a target change, a change in the nonlinear term manipulated variable UNL or the speed manipulated variable UN is discriminated, and when a change matched with the sliding mode is not shown, system abnormality is judged.