An apparatus for diagnosing bypass valve failure comprising: an electric supercharger which comprises a motor and an electric compressor, which is actuated by means of the motor so as to supply supercharged air to a combustion chamber of an engine; a bypass valve provided on a bypass line which branches from an intake line where the electric supercharger is provided, and which joins same at the front end of a throttle valve which adjusts the intake amount supplied to the combustion chamber; a vehicle-state input unit into which vehicle-state information is input; and a control unit which receives input of the vehicle-state from the vehicle-state input unit, comprising a pedal on/off state, and a fuel-cut-period entry state, so as to determine that the failure diagnosis of a bypass valve start conditions are satisfied if, while the engine is currently operating, it is confirmed that during travel the driver's pedal is in the off-state and fuel-cut-period has been entered and which, if the conditions for starting the failure diagnosis of the bypass valve are satisfied, 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, wherein when the control unit sends a first rotational speed target value and transmits an 'open' control instruction to the bypass valve and the electric supercharger is operating at the first rotational-speed target value or is operating at a value within a predetermined range using the first rotational-speed target value as a reference, the control unit monitors the amount of generated current of the electric supercharger for a first set time and if the amount of generated current of the electric supercharger has exceeded a threshold value, representing a stuck-closed failure diagnosis of the bypass valve, and the first set time has elapsed, the control unit determines that the bypass valve is in a stuck-closed failure state. a bypass valve provided on a bypass line which branches from an intake line where the electric supercharger is provided, and which joins same at the front end of a throttle valve which adjusts the intake amount supplied to the combustion chamber; a vehicle-state input unit into which vehicle-state information is input; and a control unit which receives input of the vehicle-state from the vehicle-state input unit, comprising a pedal on/off state, and a fuel-cut-period entry state, so as to determine that the failure diagnosis of a bypass valve start conditions are satisfied if, while the engine is currently operating, it is confirmed that during travel the driver's pedal is in the off-state and fuel-cut-period has been entered and which, if the conditions for starting the failure diagnosis of the bypass valve are satisfied, 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, wherein when the control unit sends a first rotational speed target value and transmits an 'open' control instruction to the bypass valve and the electric supercharger is operating at the first rotational-speed target value or is operating at a value within a predetermined range using the first rotational-speed target value as a reference, the control unit monitors the amount of generated current of the electric supercharger for a first set time and if the amount of generated current of the electric supercharger has exceeded a threshold value, representing a stuck-closed failure diagnosis of the bypass valve, and the first set time has elapsed, the control unit determines that the bypass valve is in a stuck-closed failure state.