A controller with a processing unit and a floating-point processor is disposed in a vehicle to control an actuator according to detected parameters sent from sensors. The processor generates a floating-point control parameter indicating a first quantity of control from the detected parameters at floating-point calculations, and the unit generates a fixed-point control parameter indicating a second quantity of control from the detected parameters at fixed-point calculations. The unit converts the floating-point control parameter into a converted control parameter of fixed-point representation and judges from both the converted control parameter and the fixed-point control parameter whether a failure has occurred in the processor. When no failure has occurred in the processor, the unit generates a control signal from the floating-point control parameter. The actuator is operated according to the control signal so as to give the first quantity of control to a controlled object.