PURPOSE: To improve the drivability of an engine as well as to reduce noxious exhausts, by setting an influx current, which suddenly change the output of an oxygen sensor detecting oxygen content in exhaust gases, at the specified ratio.

CONSTITUTION: An oxygen sensor 1 is alternatively supplied with a first or second influx current Is1 or Is2 from a first or second current feed device 11 or 12. The second current feed device 12 calculates a specified ratio Ki in multiplying voltage Viair and feeds the oxygen sensor 1 with the current Is2. Output Vs of the oxygen sensor 1 is suddenly changed with an air-fuel ratio commensurate to this current Is2. Doing like this, variations due to each oxygen sensor for air-fuel ratio judgement are brought to nothing whereby accurate control to the desired air-fuel ratio is performable.