The method includes the steps of: obtaining lateral magnification of an optical scanning system; obtaining the maximum value of thickness in the optical axis direction of an scanner lens; obtaining allowance b on one side and beam diameter a in the vertical scanning direction in the lens; and obtaining width h in the vertical scanning direction of the lens by the following expression h=a+2b. The allowance b is a product of the maximum value of thickness in the optical axis direction of the lens and a coefficient, and the coefficient is determined according to the lateral magnification of the system in such a way that the maximum value of movement of the focal point of the lens due to moisture absorption is made smaller than or equal to a predetermined value.