PURPOSE: To realize accurate pattern width development without being affected by irregularities in the radial direction, by a method wherein a laser light projecting means and a detecting means are set relatively movable with respect to a substrate driving means and a developing agent supplying means, in order to move a laser light irradiation position in the radial direction of a substrate.

CONSTITUTION: A substrate 1 is irradiated with laser light, and pattern width is detected from transmitted diffraction light or reflected diffraction light by using a detecting means 7. In this case, a laser light projecting means 6 and a detecting means 7 are set relatively movable with respect to a driving means 11 for the substrate 1 and a developing agent supplying means 3, in order to move a laser light irradiation position in the radial direction of the substrate 1. Hence, even when the film thickness of photoresist, the amount of exposure, and the supply amount of developing agent are not uniform in the radial direction, the pattern width is detected at all points on the substrate 1 by the scanning of the detecting means 7 regarding laser irradiation positions, thereby realizing the uniform development to be desired.