PURPOSE: To enhance the accuracy of the fuel injection amount by measuring the magnetic attraction force generated in a plunger in a provisional erecting process before a nozzle part is installed on a fuel injection valve, calculating the fuel injection amount on the basis of the result from measuring, and adjusting it to the requested value.

CONSTITUTION: When a coil 10 is fed with current, a core 2, yoke 3, and plunger 4 form a magnetic circuit, and the plunger 4 is attracted to the core 2. At the same time, a ball valve 5 in a single piece with the plunger 4 moves and separates from the seat face 6 of a nozzle 24 against the action of a spring 7, and the valve opens. As a result, the fuel, passes through a fuel passage 25 and a swirler 21 to be injected from an orifice 23. The valve opening time of the plunger 4 which determines the fuel injection amount, receives the influence of the magnetic attracting force and spring load. As provisional erecting, an appropriate spring is set on the fuel injection valve 1 previously, and the magnetic attraction force is measured. On the basis of result from measuring, the fuel injection amount is calculated, and it is adjusted to the requested value.