A control device determines whether or not the detected value of a unit conducting current output from a current sensor at least plural times is equal to or greater than a predetermined upper limit current at time of the start of the actuation of a gas sensor, and furthermore, determines that each of the units of the gas sensor is set in an abnormal state if a duration for a state in which it is determined that the detected value of the unit conducting current is equal to or greater than the predetermined upper limit current is equal to or longer than a predetermined upper limit time. On the other hand, if the duration for the state in which it is determined that the detected value of the unit conducting current is equal to or greater than the predetermined upper limit current is shorter than the predetermined upper limit time, it is determined that the detected value of the unit conducting current temporarily becomes equal to or greater than the predetermined upper limit current and each of the units is set in a normal state.