PURPOSE: To accelerate the atomization of fuel and improve combustibility by providing an air injection means near the nozzle of a fuel feed means arranged on the intake passage of an engine, and injecting compressed air from the air injection means when a wall stream of fuel near an intake port is detected by a detecting means.

CONSTITUTION: A wall stream sensor 10 is provided on an intake port opened or closed by an intake valve 2. The wall stream sensor 10 is constituted of a light emitting element generating the ultraviolet light with the preset wavelength and radiating it to a wall stream detecting point in a beam shape via a reflecting mirror and a light receiving element receiving the reflected light. The signal from the wall stream sensor 10 is guided to a photo-electric converter 42 via a signal line passing a wall stream sensor guide 41 formed on a cylinder head 42, and the converted electric signal is inputted to a control unit 43. When the occurrence of the wall stream is detected here, a motor-driven air pump 44 is driven, compressed air is injected from an air feed passage 45 arranged near the nozzle of an injector 9, and the atomization of fuel is accelerated.