PURPOSE: To improve combustion in an engine so as to reduce the exhaust of nitrogen oxide by enabling main injection and sub injection in both the prescribed timing and the injection rate without pushing up cost and without increasing noise.

CONSTITUTION: A free piston 27 energized upward by means of a pressure spring 26 is arranged so as to be movable downward by a prescribed distance (d). When fuel is fed to a fuel inlet port 20a and a fuel introduction passage 20b under pressure in its early stages of fuel feed under pressure, only when the second fuel introduction passage 22 and a feed hole 23 are communicated with each other before the free piston 27 moves downward, the fuel is supplied to a fuel reserving chamber 18, and a needle valve 17 moves upward, and sub injection is carried out. Further, when fuel pressure is increased and the free piston 27 moves downward by a distance (d), the first fuel passage 21 comes to communicate with the feed hole 23 so that main injection can be carried out.