PURPOSE: To enable easy positioning with high accuracy with a laser device which irradiates laser light through a prescribed optical guide path onto a substrate and providing means for decreasing the copy lens of the laser light to the optical guide path.

CONSTITUTION: The laser light from a laser oscillator 1 is passed through a deflection prism 2 and a quarter-wave plate 3 and after the direction thereof is changed by a prism 4 on an X-stage 18, the light is conducted onto a Y stage 19 and passes through an aperture 5. About 99% laser light is reflected by a half mirror 6 and is irradiated via a mechanical shutter 7 and a projecting lens 8 onto the silicon substrate 10 in a chamber 9. The control (Z-axis direction) of the position and spot diameter of the laser light on the substrate 10 is accomplished by conducting the light transmitted through the mirror 6 and reflected from the substrate 10 to the 1st mirror 11, a condenser lens 12, the 2nd mirror 13 and a microscope lens 14 and detecting the pattern thereof, i.e., the pattern of a positioning mark with a television camera 15. A scattering plate 20 is provided between the mirror 6 and the prism 4 in the stage of positioning in this case.