PURPOSE: To carry out feed back control of a proper air fuel ratio by changing the opening degree of a control valve by only a specified step value according to a cooling water temperature, in cold acceleration operating condition of an internal combustion engine.

CONSTITUTION: In the case where the cooling water temperature of an internal combustion engine 2 exceeds either one of a plurality of set temperature values and the throttle valve 12 of a carburetor 10 is opened to a larger degree than an idle opening degree by a control means 26, the opening degree of a control valve 22 is changed once by only a specified step value according to a cooling water temperature by an actuator 24. And then, the opening degree is changed per prescribed step value on the basis of the detected signal of an exhaust gas sensor so as to make feed back control so that an air fuel ratio may be set to a target value. It is, thus possible to change the opening degree of the control valve 22 by only the specified step value according to the cooling water temperature, in clod acceleration operating condition of the internal combustion eugine 2.