PURPOSE: To prevent the suspension of air-fuel ratio control through inactive judgement by setting the standard value in the air-fuel ratio feedback control for the cylinder group having the higher cooling degree to the smaller value than that of the cylinder group having the lower cooling degree, in an engine equipped with a plurality of cylinder groups such as V-type engine, etc.

CONSTITUTION: In a V-type 6-cylinder engine 1 placed horizontally, cylinders are divided into two groups consisting of the rear bank 1R of the cylinders #1W#3 and the front bank 1F of the cylinders #4W#6, and O2 sensors 5 and 6 are installed into exhaust pipes 2 and 3, respectively, and air-fuel ratio feedback control is carried out for each cylinder group by an electronic control unit 7. The degree of cooling is high on the front bank 1F side, since the cooling wind of a radiator fan, etc. is received, and the temperature of the O2 sensor 6 becomes lower than that of the O2 sensor 5, and the output voltage is increased. The standard value for the air-fuel ratio control on the front bank 1F side is set smaller than that on the rear bank side, and the low output side level does not increase from an inactive judgement value.