PURPOSE: To reduce the accuracy of chromatic aberration and eliminate its difficulty, and to reduce the cost by providing a means which vary the wavelength of the 2nd light within a specific range so as to further correct the chromatic aberration generated between the 1st light and the 2nd light owing to the correction accuracy of a refraction optical system.

CONSTITUTION: An excimer laser 1 as a light source for printing is a KrF laser which oscillates almost at, for example, 248.5nm and a wavelength varying means 9 varies the wavelength of output light within a range of original oscillation beam width of about 1nm (i.e. 248.5±0.5nm) by slanting a wavelength selecting element such as a prism and a grating arranged in the cavity of the laser so as to narrow down the band. An illumination optical system 2 irradiates a reticle uniformly with laser light from the excimer laser 1 at a certain specific spread angle. An alignment optical system 3 uses an He-Ne laser 8 which oscillates with signal wavelength of, for example, 632.8nm as a light source and observes and measures the position relation between an alignment mark on the reticle 4 and an alignment mark on a wafer 6 observed through a projection optical system 5.