PURPOSE: To optimally control the fuel injection rate by arranging a valve system in the way of an injection path between a fuel injection pump and an injection valve while varying the opening of the valve system with correspondence to the engine operating condition thereby varying the effective area of the injection path.

CONSTITUTION: The delivery port of a fuel injection pump 1 and the injection valve 2 mounted on respective combustion chamber are coupled through an injection path 3. A spool valve 4 is arranged in the injection path 3 where the spool 5 is moved to the right and left through a cam device 6 thus to vary the opening of said valve 4 and to vary the effective area of the injection path 3. While the detected signals of engine rotation and load are fed to a controller 7 to vary the driving signal to be fed to a cam device 6 with correspondence to the operating condition of engine. Consequently, when varying the opening of spool valve 4 with correspondence to the engine operating condition, the optimal injection rate can be obtained at any time.