PROBLEM TO BE SOLVED: To precisely detect stress of a sample semiconductor wafer, as the absolute value, without rotating the sample or the overall optical system.
SOLUTION: A birefringent phase difference is generated when a laser light R is modulated by photo elastic modulation by a PEM (photo elastic modulator) 6, and is detected after passing through a first and a second 1/4 wave plates. The reference signal data thereof are stored in a signal processor. The laser light R of the polarized wave, modulated by the photo elastic modulation by the PEM 6 and passing through the 1/4 wave plates, has the birefringent phase difference, and is transmitted through the semiconductor wafer D having the residual stress. When transmitted through a test piece, the direction of the stress of the test piece is detected when the angle with respect to the linearly polarized light is 0 degrees and 90 degrees. The transmission electric signal thereof is transmitted to an analog/digital converter 16. The signal is inputted into the signal processor, where the transmission signal is generated. The signal processor reads out the stored reference signal data and the transmission signal data, and calculates the reference birefringent phase difference and the absolute value of the birefringent phase difference.