PURPOSE: To enable detection of defects in a liquid-crystal mask, by providing a polarizing filter between a laser oscillator and the liquid-crystal mask, provid ing a beam splitter between the mask and a projection lens to branch a laser beam, and picking up an image by a television camera.

CONSTITUTION: A laser beam from a laser oscillator 1 is polarized directly by a polarizing plate 2, and is projected through a liquid-crystal mask 4. The mask 4 is provided with a required printing pattern by a liquid crystal controller 8, and the laser beam is transmitted, without changing the plane of polarization, through the area provided with the printing pattern. The laser beam transmitted through areas other than the printing pattern area of the mask 4 has the plane of polarization rotated by 90° upon the transmission, and is then separated through reflection by a beam splitter 5. The rotated laser beam 16B reflected by the beam splitter 5 is attenuated by an attenuator 10, and is picked up as an inverted image by a television camera 13. The image is binary coded by an image recognition device 15, and is compared with an image obtained under a normal condition, whereby an abnormality in the mask 4 is detected.