To solve the problem of how to eliminate a useless tact and improve alignment accuracy in a process of relatively aligning a light emitting element with optical parts.

In this optical module assembly method, data on an optical axis and a spreading angle of light necessary for arranging a light emitting element 1 in ideally positional relation to an optical part 2 which is used for making light rays projected from the light emitting element 1 parallel or focused are obtained through an FFP optical measurement system capable of measuring the optical axis and spreading angle of light without changing a distance between the element 1 and the optical part 2. An optical module assembly device is equipped with a stage 4 on which the light emitting element 1 can be mounted, a hand capable of grasping the optical part 2 and moving it in a uniaxial direction, and the FFP optical measurement system 3.