The invention provides an optical module, an image reader and an assembling method of an optical module. The optical module 100 includes an optical path unit 101 including a plurality of reflectors 104 for securing an optical path of reflected light from a manuscript, an image processing unit 102 including a reading device 105 which reads image information on the manuscript based on the reflected light from the manuscript entered via the optical path, and a connecting component 103 which connects the optical path unit 101 and the image processing unit 102 so that their positional relation will become a prescribed state.