PURPOSE: To purify exhaust gas of a Diesel engine and prevent deterioration of the performance by a method wherein two kinds of small-diameter sections and a collar are fitted on top of the plunger of a fuel injection pump of a Diesel engine to allow a two-step fuel injection.

CONSTITUTION: A small-diameter section 41, a collar 42 and another small-diameter section 43 are fitted on top of a plunger 4 of a fuel injection pump. The outer diameter d1 of the small-diameter section 41 must be about 0.75DW0.975D with respect to the maximum outer diameter D of plunger 4. Furthermore, the outer diameter d2 of collar 42, the diameter and length of the small-diameter section 43 are set based on an initial injection rate and other references. With these settings, small-diameter section 41 slightly catches a fuel support port 3 at an initial injection, so that the injection arises at a low pressure. At finish of the initial injection, collar 42 catches the port 3 so that the injection is interrupted, and then when the small- diameter section 43 just passaed the port 3 the main phase of injection is carried out at a high pressure.