PURPOSE: To measure the internal stress distribution in a body to be measured which has a uniform shape and stress distribution in one direction at a high speed without destruction by measuring the optical path difference of light transmitted through the object body due to opto-elastic effect while rotating the object body.

CONSTITUTION: The body 4 to be measured is fixed on a sample turntable 5, whose angle of rotation is set to some angle. In this state, laser light from a laser 1 which is polarized linearly by a polarizer 3 is allowed to strike the body 4 to be measured. This incident light is split into two components having mutually orthogonal planes of polarization by birefringence originating from the stress distribution in the body and they travel at different propagation speeds, so light passed through the body has an optical path difference and becomes an elliptic polarized wave. Further, the object body 4 has a uniform and stress in a direction Z and the light beam having this optical path difference is passed through a quarter-wavelength plate 7 to become linear polarized light. For the purpose, the optical axis of an analyzer 8 is rotated to read data on a vidicon 9 in an image memory 10 and the optical path difference is calculated by using a computer 11. Thus, a measurement is taken easily and precisely without destruction.