A scanning exposure method moves a mask and a substrate synchronously. A first mask and a second mask are provided for a mask stage. The mask stage has a reflective surface extending in a scanning direction, and the first and second masks are arranged along the scanning direction. Surface curvature data is prepared on the reflective surface in relation to positions of the first and second masks. Positional information of the mask stage is detected by irradiating the reflective surface with a measuring beam. Then, the mask stage is moved based on the surface curvature data and the detected positional information in a direction other than the scanning direction.