In a measurement method for measuring polarization characteristics in which an image of a mask pattern is projected onto an image plane, a first and second slit having a width less than or equal to the wavelength of a light source are displaced on the image plane and light passing through the first and second slit is detected to obtain a first and second light intensity distribution with respect to the direction of displacement of the first and second slit. The positions at which the first light intensity distribution takes a maximum and a minimum value are determined. An index value is calculated using the respective light intensities in the second light intensity distribution at positions corresponding to the determined maximum and minimum positions. Polarization characteristics corresponding to the calculated index value are obtained by using information expressing the relationship between the index value and the polarization characteristics.