PURPOSE: To make it possible to align the position of exposure without passing light through a projecting lens, by projecting light comprising a plurality of wavelengths on an aligning mark on a substrate obliquely from the side of a gap between the reduction projecting lens and the substrate, detecting diffracted light from the aligning mark with a detector, and positioning the aligning mark at a specified place.

CONSTITUTION: P polarized laser having four wavelengths is projected on a substrate 4 at a lighting angle θ of about 75°. A linear dot pattern which generates diffraction is used as an aligning mark 65, and diffracted light 62 is obtained. The diffracted light is condensed through an objective lens 112. The real image is formed on a diffraction grating 114 under the P polarized state. The diffracted light 62 is further condensed through intermediate lenses 115 and 117. Thus a second real image 118 is formed and transduced into an image signal through an optoelectronic transducer 119. The position of the aligning mark is detected at an exposing position. Positioning is performed at a specified place. In this way, the position of the substrate can be determined at high sensitivity and high accuracy at the exposing position without interposing a reduction projecting lens.