PURPOSE: To diagnose an abnormal condition of an O2 sensor itself securely by detecting the O2 sensor output in a heater OFF condition and the O2 sensor output in a heater ON condition of a heater to heat the O2 sensor respectively, and comparing the both outputs.

CONSTITUTION: Since the exhaust temperature is reduced suddenly in a speed reduction condition of an engine, an O2 sensor 1 also receives a cooling operation, and when a fuel cut is carried out in the speed reduction, the air-fuel ratio is made in a lean condition. In consideration of such a point, a heater converting means 4 to make a heater 2 heat the O2 sensor 1 in the OFF condition and the ON condition in an adequate time respectively when a speed reduction detecting means 3 detects that the engine is in a speed reduction condition accompanied by a fuel cut is provided, and the O2 sensor outputs in a heater OFF condition and in a heater ON condition are detected by output detecting means 5 and 6 in the heater OFF time and in the heater ON time. And the both outputs are compared, and when its difference or its ratio is small, it is decided to be abnormal by a deciding means 7.