Optical birefringence is measured by a high-frequency modulation method using a photoelastic modulator and a polarized laser. This measurement has the high sensitivity required to measure very small birefringence produced by stress. By this measuring method, the magnitudes of the differences in principal stresses and their directions are obtained directly and quantitatively. The photoelastic properties of glass, polymethyl methacrylate (PMMA) and two kinds of epoxy resins are measured with the equipment. The distributions of optical birefringence corresponding to stress distributions are measured for glass plates bonded with epoxy resin. The residual stress caused by the bonding process is measured. This stress measuring method is applicable to detection of the stress distribution of transparent solid materials.