PURPOSE: To reduce the occurrence of a fluctuation in a fuel injection rate during specially low speed low load running, by a method wherein, after completion of injection of fuel, the pressure of a main injection chamber is held at a specified value equal to that of a constant pressure chamber irrespective of an operating condition, and a stable and specified fuel remaining pressure is produced.

CONSTITUTION: By applying the pressure of fuel on a plunger 11 of a variable injection rate mechanism 10 through a pipe 36 with the aid of a pump 37, the plunger 11 is lowered against a spring 19. Fuel in a main injection chamber 15 is escaped to the constant pressure chamber 41 side, and fuel in a servo chamber 16 is fed with a pressure to a nozzle body 34. When the plunger 11 is further lowered, fuel in the main injection chamber 15 is also fed with a pressure to the nozzle body 34 through the servo chamber 16, and main injection takes place. Thereafter, when the plunger 11 is raised, fuel in the constant pressure chamber 41 is sucked to the servo chamber 16 and the main injection chamber 15, when the plunger 11 is further raised, the main injection chamber 15 is shut off from the servo chamber 16 and meanwhile the main injection chamber is communicated with the constant pressure chamber 41, and the pressure in the main injection chamber is regulated to a value equal to that of the constant pressure chamber 41.