To effectively reduce a measurement error in a parameter indicating two-dimensional spatial distribution of a state of polarization generated by variations in retardation of a birefringent prism pair due to a temperature change or other factors, while holding a variety of properties of an imaging polarimetry using the birefringent prism pair. By noting that reference phase functions .phi..sub.1(x, y) and .phi..sub.2(x, y) are obtained by solving an equation from each vibration component contained in an intensity distribution I(x, y), the reference phase functions .phi..sub.1(x, y) and .phi..sub.2(x, y) are calibrated concurrently with measurement of two-dimensional spatial distribution S.sub.0(x, y), S.sub.1(x, y), S.sub.2(x, y), and S.sub.3(x, y) of Stokes parameters.