PURPOSE: To improve mixing performance, reduce generation of smoke, reduce fuel consumption, and increase output, by recessedly forming a fine vortex flow generating groove on the inner circumferential wall of a vortex chamber so as to cross the direction of push-in vortex flow.

CONSTITUTION: On the inner circumferential wall 2a of a vortex chamber mouth piece 10, a fine vortex generating groove 6 is recessedly formed on a little upper side of a nozzle hole 3, in the horizontal direction, by spot facing work. A fuel injection nozzle 7 is fitted to the upper part of a vortex chamber 2, so that the main flow of fuel infected from the fuel injection nozzle 7 is faced to the nozzle hole 3 and a part is faced to the fine vortex generating groove 6. A part of injected fuel from the fuel injection nozzle 7 is blown against the fine vortex generating groove 6, and fuel spray particles are favorably caught in fine vortex flow 11. In this way, mixing performance is improved, and combustion performance can be also improved.