The whole-field measurement of photoelasticity using linearly polarized light is performed from a method measuring the phase of the object wave by rotating of a quarter-wave plate and a linear polarizer. The paper studies the accuracy of the method by considering the variation of the incident light intensity and the background intensity. The magnitude of variation of the light intensities is expressed by using random numbers. The simulation for the relative phase retardation ρi and the directions of principal stresses Ψ in the circular disk under diametral compression load is carried out. In the results of simulation using the intensity variation with random numbers, ρi and Ψ are found to be accurate by using this measuring method.