PURPOSE: To provide a fuel injection device capable of stabilizing the number of idle revolutions by increasing a fuel increase amount so as to keep a combustion air-fuel ratio at a constant value after cut off of assist air.

CONSTITUTION: The opening time of a fuel injection valve 100 is controlled by a control means 105 according to a given control program based on various detection signals for an engine cooling water temperature, the number of revolutions of an engine, and an amount of intake air, and an opening closing control valve 103 to cut off air fed to a fuel atomizing means 101 is controlled by the same control means. A fuel increase means 106 increases an amount of liquid fuel fed to the fuel injection valve 100 according to opening and closing of the opening closing control valve 103 and an engine cooling water temperature. Further, when the opening closing control valve 103 is closed and an engine is in a steady operation state, an increase value of liquid fuel fed to the fuel injection valve 100 can be gradually increased by a fuel quantity increase means 106a.