A constituent parts assembling method for an actuating apparatus includes a step of assigning ID classification to each of constituent parts with reference to characteristics difference of respective parts when these constituent parts give influence to output characteristics of an injector. In the processes of assembling these constituent parts, when a selected constituent part has a certain ID classification being assigned beforehand, this assembling method includes a step of executing constituent part selection in accordance with an instruction of constituent part designating means which designates other constituent part having an ID classification corresponding to the ID classification assigned to the selected constituent part, and also includes a step of assembling the selected constituent parts into the injector.