In a small-sized optical module, opposite side surfaces of a collimator lens (2) and opposite side surfaces of a polarization compensating filter (5) are fixed to opposite side surfaces of a rectangular frame (4). Two open surfaces are provided in the rectangular frame (4), so that optical components can be finely adjusted easily, and a large number of small-sized optical components can be aligned and assembled accurately. Reinforcing members (6) are provided in the open surfaces of the rectangular frame (4) so that the shape of the rectangular frame (4) can be retained against external force. After a diffraction grating (3) is fixed to the reinforcing members (6), the reinforcing members (6) with the diffraction grating (3) are adjusted, aligned and fixed to the rectangular frame (4).