PURPOSE: To prevent freezing of a throttle valve when it is cold, and to restrain engine stop by circulating exhaust gas to the upstream side of the throttle valve to heat intake air in the case of no-load or light load where the radiation quantity of an engine is small, and freezing of the throttle valve is easy to occur.

CONSTITUTION: An exhaust gas circulating path 9 is derived from an exhaust passage 5, the drived end of the exhaust gas circulating path 9 is connected to the upstream part of a throttle valve 23, an opening and closing valve 8 is installed in the midway of the exhaust gas circulating passage 9, and the opening and closing valve 8 is linked with a load detecting means 10. If the engine load is a designated preset value or less, the opening and closing valve 8 is set to the opening state, and if the engine load exceeds a designated preset value, the opening and closing valve 8 is set to the closing state.