1. A non-transitory computer readable medium storing a program to be installed in a computer for processing measured data obtained by polarization-sensitive optical coherence tomography (PS-OCT), said program characterized in that it causes the computer to function as: a means for creating and storing multiple sets of phase difference distribution data for different effective signal-to-noise ratios (ESNRs) beforehand, where each set comprises a distribution corresponding to each of multiple phase constants from 0 to π; a means for determining a coefficient of non-linear conversion function for converting phase difference distribution data measured by PS-OCT to symmetrical phase difference distribution data; a means for estimating an ESNR from phase difference distribution data measured by PS-OCT and then using the estimated ESNR to select one of the multiple sets of phase difference distributions created for different ESNRs; a means for converting phase difference distribution data measured by PS-OCT to symmetrical phase difference distribution data, by using the conversion function based on the coefficient determined according to the selected set of phase difference distribution; and a means for obtaining a true phase constant by averaging the symmetrical phase difference distribution data, wherein the non-linear conversion function conforms to Equation (2) below; where, in Equation (2), δE is a symmetrical phase difference distribution, δM is a phase constant measured by PS-OCT, bi (i=0, 1, 2, 3,. .. , n) is a coefficient of conversion function, and γ is an ESNR, δ E = ∑ i = 0 n ⁢ b i ⁡ ( γ ) ⁢ δ M i = b 0 ⁡ ( γ ) + b 1 ⁡ ( γ ) ⁢ δ M + b 2 ⁡ ( γ ) ⁢ δ M 2 + … + b n ⁡ ( γ ) ⁢ δ M n Equation ⁢ ⁢ ( 2 ) wherein γ is defined as follows: 
 1/γ=1/4(1 /SNR S,0 +1/SNR S,1 +1 /SNR r,0 +1 /SNR r,1 ) wherein s and r represent a sample arm and reference arm of PS-OCT, respectively, and 0 and 1 each represent the number of times, respectively.