PURPOSE: To realize an air assist system capable of demonstrating the effect even in the starting.

CONSTITUTION: In a fuel injection device of an internal combustion engine provided with a pressure difference type air assist system, a pressurized gas cylinder 11 is connected to an air assist passage 6 through a control valve and a pressure regulating valve 10, and the pressurized gas is introduced into the air assist passage 6 by releasing the control valve when the engine is started, the engine is warmed up, or the engine is driven with high load. In the fuel injection device of the internal combustion engine provided with the pressurization type air assist system, the pressurized gas cylinder 11 is connected to the air assist passage 6 through the control valve and the pressure regulating valve 10, the pressurized gas is introduced into the air assist passage by releasing the control valve when the engine is started or warmed up, and the pressurized air is introduced into the air assist passage after warming-up. The pressurized gas can be the fuel gas.