A constant current circuit is controlled to stop a constant current Ics (Ics=0) flowing between sensor electrodes of an oxygen sensor, in a period before the oxygen sensor becomes active, during a fuel cut of an engine, and after the engine stops. When an abnormality (for example, a failure) occurs in the constant current circuit, a sensor output equivalent to an output of the oxygen sensor of when the constant current circuit is controlled to stop the constant current Ics is different from a normal sensor output. An abnormality diagnosis is executed to determine a presence or absence of the abnormality in the constant current circuit depending on whether the output of the oxygen sensor is out of a predetermined normal range, in a period before the oxygen sensor becomes active, during a fuel cut of an engine, and after the engine stops (when the constant current circuit is controlled to stop the constant current Ics).