A phase plate is superposed on a sample, and this phase plate is so adjusted that the phase difference of the total retardation of the sample and the phase plate is integral times 2.pi. with respect to a measuring beam of a first wavelength, so that retardation can be correctly measured even if an order is increased. In this state, a measuring beam of a second wavelength which is approximate to the first wavelength is employed and two polarizing plates maintaining polarizing directions in parallel nicol relation are singularly rotated with respect to the sample which is arranged therebetween. The ratio Im/Io between maximum value Io and minimum value Im of currently transmitted light intensity is applied to a previously prepared relation between the order n of retardation and this ratio Im/Io to derive the order of retardation of the sample, to thereafter obtain correct retardation.