This invention relates to an assembly method, for head units mounting a plurality of head chips for discharging ink onto a frame, in which relative positions among head chips are determined and which the head chips are secured onto the frame so as to maintain the relative positions, to an assembly apparatus for head units based on the method, and further to an ink jet output apparatus incorporating those head units. According to this invention, correction by each image output will not be required because the head unit is able to complete adjustments of shifts of arrival positions of ink at its assembling stage, thereby making the apparatus compact and simple. In addition, correction of positional shifts will not be required when the heads are replaced, so that maintenance of the ink jet output apparatus will be easy.