The invention relates to a method of assembling a needleless injection device, whereby a plurality of elements forming a circuit of elements is assembled to a body. Advantageously, this is done to avoid buffer overflow associated with ongoing NCP, and the delay associated with SAW. Such an arrangement ensures that a valve-operating system can be constructed compactly, and the opening lift amount of the engine valve can be changed continuously, while ensuring an excellent followability to the valve-operating cam. Thus, a desired value can be always generated while reducing the number of fuse circuits blown.