In order to reduce mask pattern transfer error caused by expansion of the mask substrate during transferring the circuit pattern onto the photosensitive substrate, a mask substrate is loosely supported on a plurality of mounts on the mask stage so that the mask substrate can freely expand in response to changes in its temperature. A measuring instrument (such as a temperature sensor or an interferometer) is used to measure a value representing the expansion amount of the mask substrate. Alignment and positioning of the mask substrate and the photosensitive substrate is adjusted in response to the expansion amount of the reticle, based on the measured value.