To detect the breakage, etc., of a return spring to return a valve to a default opening in a system using an electronically controlled throttle valve.

In the system comprising a valve 11 for opening/closing a suction passage 1a, a DC motor 16 for driving the valve 11, a return spring for returning the valve 11 to the default opening when a DC motor 16 is not in operation, the DC motor 16 detects the actual throttle opening of the valve 11 at the time of non-actuation by a throttle opening sensor 20, a control part 110 determines whether the valve is in the default opening or not in accordance with the detected information and an alarm device 170 generates an alarm signal in the case when it is not in the default opening. Consequently, it is possible for an operator to recognize failure and to carry out a desired countermeasure such as repairing, etc.