a bypass valve on a bypass line branching from an intake line with the electric supercharger, wherein the bypass valve joins the intake line at the front end of a throttle valve for adjusting an intake amount supplied to the combustion chamber;

a vehicle-state input unit receiving vehicle-state information; and

a control unit which assesses the vehicle-state information to determine whether failure criteria of a bypass valve start are satisfied; and

if the failure criteria are satisfied, the control unit transmits ‘open’ or ‘close’ control instructions to the bypass valve and transmits, to the electric supercharger, a specific rotational-speed target value for determining whether the bypass valve is in the normal open state or the normal closed state, thereby determining whether the bypass valve is subject to stuck-open failure or stuck-closed failure.