PURPOSE: To reduce the volume of exhaust gas by controlling air/fuel ratio according to the air/fuel ratio read out according to an engine temperature established by different air/fuel ratio schedules while transmission is in drive and while transmission is in neutral.

CONSTITUTION: The above mentioned device controls the air/fuel ratio by means of calculating the fuel injecting volume by an engine controller 16 in accordance with various engine parameters and controlling an injector 14 when an engine 10 is in operation. In this case, a ROM is provided in the engine controller 16, for storing as respective engine parameters a first and second air/fuel ratio schedules indicating the desired air/fuel ratios of the mixed gas while transmission is in drive and while transmission is in neutral. One of the air/fuel ratio schedules is selected in correspondence with the state of the transmission detected by a switch means 18. The air/fuel ratio is set up according to the engine temperature and fuel injecting volume is controlled accordingly.