PURPOSE: To promote uniform atomization of fuel and to thereby improve ignitability of fuel, by providing a booster chamber having therein a booster, an accumulator and a fuel chamber disposed between the booster chamber and the accumulator.

CONSTITUTION: A booster 12 is disposed in a booster chamber 11 which is communicated with an air source A via a check valve V1 and also communicated with a fuel chamber 13 via a check valve V2. The fuel chamber 13 is in turn communicated with an accumulator 14 via a check valve V3. During the suction stroke of the booster 12, air from the air source A is drawn into the booster chamber 11 through the check valve V1. At the time of the compression stroke of the booster 12, air in the booster chamber 11 is introduced into the fuel chamber 13 through the check valve V2. Further, at the end of the compression stroke, a check valve V4 is opened for injecting fuel into the accumulator 14 together with air. Since, with such an arrangement, uniform atomization of fuel can be promoted, it is enabled to improve ignitability of fuel.