PURPOSE: To realize a nozzle performance adjusting method for a fuel injection nozzle, capable of shortening the adjusting time, and capable of performing the adjustment of nozzle performance by a simple facility at low cost.

CONSTITUTION: Processing oil containing abrasive grains is supplied from a piston pump 15 into a fuel injection nozzle 1 by the constant discharge pressure by the rotational drive of a motor of a driving unit 24, a nozzle hole 2 of the nozzle 1 is so processed that the passage sectional area S may be enlarged. A CPU 26 of a controller 25 calculates the flow rate Q of processing oil on the basis of the detected signal from a position detecting sensor 28 for detecting the position of a piston rod 23. When the flow rate Q becomes the target flow rate QT prestored in a memory 27, the CPU 26 stops rotational drive of the motor, and stops supply of processing oil from the piston pump 15 to the fuel injection nozzle 1.