PURPOSE: To enhance function of a 3-dimensional catalyst by forming a by-pass between the suction passage at the upper stream of air flow meter and the rest side suction passage at the understream of the fresh air shutoff valve and thereby introducing a small amount fresh air at the time of light load and an ample amount of fresh air into the resting cylinders at the time of idling.

CONSTITUTION: During partial cylindric operation, in which fuel supply to the resting cylinders #4W6 is cut, the fresh air shutoff valve 5 in the rest side suction passage 4 will be closed, when the control valve 23 in the by-pass passage 22 is opened in the light load range in accordance with the negative pressure for intake to the engine. In this case, the degree of opening of the control valve 23 is comparatively small, so that a small amount of fresh air is introduced into the rest side cylinders #4W6. Accordingly the pumping loss in the cylinders #4W6 is reduced as well as the exhaust air therefrom eliminates substantially drop of temp. of the combustion exhaust gas out of the working side cylinders #1W3. Consequently the function of the three-dimensional catalyst 18 is enhanced to ensure exhaust of a purer exhaust gas.