To improve a photosensitive substrate in uniformity of exposure and illuminance, when an image of a reticule pattern is projected onto a photoreceptive substrate through a slit scan exposure method through the use of an optical pulse source.

A slit-illuminated region 24 is illuminated with optical pulses emitted from an optical pulse source 1 through illuminating optical systems 2 to 10, and an image present in the illuminated region 24 is projected to an exposure region 24W through a projection optical system 15. A reticule R and a wafer W are scanned in the opposite directions making them synchronized with each other, whereby the pattern of the reticule R is transferred onto the wafer W. Emission trigger signals TP of constant frequency are supplied to the optical pulse source 1, whereby the number of exposure light pulses is determined by taking into consideration the dispersion of light pulses in light volume and light emission timing of the light pulse source 1.