PURPOSE: To expedite atomization of fuel by adjusting the opening of a valve set in a passage for bypassing the upper part and the lower part of a supercharger, feedback-controlling it to be a target supercharging pressure, and supplying assist air in the vicinity of a fuel injection valve, at the time of cold operation of an engine.

CONSTITUTION: A part of intake air is taken from an intake passage 8 between a throttle valve 10 and a supercharger 12 in the case of an engine 1, and an assist air passage 16 for supplying it in the vicinity of an injection nozzle of a fuel injection valve 15 is provided as assist air for expediting atomization of the fuel. In this case, opening of a bypass valve 24 set in the passage 23 for bypassing the upper and lower parts of the supercharger 12 is regulated and it is feedback-controlled so that supercharge pressure by the supercharger 12 may becomes the target supercharge pressure at the time of cold operation of cooling the engine 1. The assist air is supplied via the assist air passage 16 in the vicinity of an injection nozzle of the fuel injection valve 15. Consequently, atomization of fuel is expedited even at the time of cold operation of the engine.