To accurately correct the deformation component of a moving mirror on a wafer stage measure the curving data of the moving mirrors prior to the installation onto the wafer stage and store the data as a function of the distance from one end of the mirror. Then, measure the discrete curving error data of the moving mirrors after installation onto the wafer stage at every distance, d, using two laser interferometers arranged in parallel at a distance, d. Store the measurement results in the memory. Finally, a main controller creates a continuous curving error data of the moving mirror after installation onto the wafer stage based on the relationship between curving data and curving error data of the moving mirror before and after installation onto the above wafer stage to use it as the correction data.