PURPOSE: To measure surface stress accurately, by a method wherein a laser beam is excited to run in parallel with a surface of an object to be measured, and surface stress is measured by utilizing a senarmount compensator according to the difference of photoelastic beam pathes arisen in the emitted beam.

CONSTITUTION: A laser beam 8 emitted from a light source 29 is reflected at a prism 38 and reached to a glaze layer A of an object B to be measured. The beam further advances in the layer A, and then, a part of the beam passes through an exit prism 26, and through a quarter wave plate 43 which forms a senarmount compensator with a polarizing plate 32B, and is detected by a photodetector plate 47. When there is stress, it is observed as the contracted part 316 in a streak of light 313 by rotating the plate 47. The stress can be measured on the basis of the Senarmount law, by measuring the shift of the shadowed part 311 which is shifted as the plate 47 rotates.