A lean air/fuel ratio control device is provided for feeding the engine with a mixture of a desired lean air/fuel ratio for a given time from the time on which the engine is started. A stoichiometric air/fuel ratio control device is provided for feeding the engine with a mixture of a substantially stoichiometric air/fuel ratio once the given time passes. A temperature sensor is provided for sensing the temperature of cooling water of the engine at the time when the engine is just started. A control unit is provided for varying the given time in accordance with the temperature sensed by the temperature sensor. The given time may be varied in accordance with the activity of the oxygen sensor or the temperature of exhaust gas from the engine.