To provide an aligner by which desired exposure can be performed under optimum conditions by minimizing the out-of-focus exposure, even when the flatness of a semiconductor wafer partially varies.

The flatness of each one-chip-size region 15A on a semiconductor wafer is measured. A virtual plane 28 is determined in each region according to the measured flatness information. When the region 15A is exposed to light in region units, a stage 16 for exposure is tilted so as to obtain an optimal angle between a normal 29 on the virtual plane 28 and an optical axis 30 of an aligner optical system.