There is provided an assembling method for expanding a maximum admissible astigmatism of objective lens which includes defining a maximum admissible astigmatism for the entire optical system, installing the collimator lens on the optical system to cause a predetermined amount of astigmatism lying within a range of up to the maximum admissible astigmatism by tilting the collimator lens from a condition where a point source of the semiconductor laser lies on an optical axis of the collimator lens, and installing the objective lens on the body case of the optical system to bring a total amount of astigmatism of the entire optical system to a level smaller than or equal to the maximum admissible astigmatism for the entire optical system by rotating the objective lens from a condition where the point source of the semiconductor laser and the center of the collimator lens lie on the optical axis of the objective lens.