To measure the thermal expansion quantity of an original plate, with out affecting the throughput of the aligner, by measuring the elongation of the original plate, controlling the scale factor of a projection optical system according to the elongation, and projecting an image of the plate on a board through the projection optical system.

Measuring optical systems 7L, 7R measure the positions of reticle marks 5L, 5R written on a reticle, relative to two reference marks 4A, 4L fixed to the top of a projection optical system 3. Temp. conditioners 8L, 8R control the ambient temp. The reference marks 4L, 4R fixed to the optical system 3 make the reference mark distance 4L-4R const. and hence provides the elongation ΔW of the reticle at time such that ΔW=W(t)-W(0), W(t)=L(t)-R(t)/4L-4R. The reticle mark distance is measured every wafer to correct the projection scale factor.