To provide an assembly method and device for an optical device and the optical device for shortening the time required for assembly.

In this assembly method for a light source unit 1 having optical elements 12, 13 and SIDM devices 21, 22 for moving the optical elements 12, 13, respectively, cameras 23a-23c for photographing optical element 12, 13 sides are arranged at positions offset for optical axes of the optical elements 12, 13, finished images indicating the positional relationship between members of the light source unit 1 when viewed in the direction of photographing from the positions of the cameras 23a-23c in the light source unit 1 in a completed condition are prepared in advance, the positions of the optical elements 12, 13 are detected by the images photographed by the cameras 23a-23c, and the optical elements 12, 13 are arranged by matching them with the finished images by the SIDM devices 21, 22 while comparing the photographed images with the finished images.