PURPOSE: To shorten the time required for strain measurement by investigating the plane shape of a work without contact and without scanning and automatically straighter the strain of the panel plate in accordance with the straightening data obtd. from the result of the measurement.

CONSTITUTION: The moire fringes of a surface 5a to be measured generated by the correlative effect of a grid 3 for allowing the transmission of the light projected from a light source 2 of an illuminating device 1 and irradiating the surface to be measured with the transmitted light and a grid 8 for passing the light reflected by the surface 5a to be measured are picked up by an image pickup device 6. The images are converted to image signals which are applied to an arithmetic section 10. This arithmetic section 10 measures the three- dimensional shape of the surface 5a to be measured in accordance with the image signals corresponding to the moire fringes, computes the straightening point and the straightening quantity and outputs the straightening data to a controller 11. The controller 11 outputs the straightening signal to a straightening section 12 in accordance with the straightening data. The stress straightening of the work is thus automatically executed.