In a step S1, an excimer laser control unit sets a central wavelength command value, an integral spectrum distribution command value, and an exposure pulse number in a laser internal control unit. In a step S2, the laser internal control unit receives the central wavelength command value, the integral spectrum distribution command value, and the exposure pulse number from the excimer control unit, and executes an actual exposure when an exposure command is received. In a step S3, the laser internal control unit, after executing exposure, measures the central wavelength of the pulse and the spectral distribution, and corrects any discrepancy between the actually measured central wavelength and the central wavelength command value by outputting a drive command to the narrow-band module so that the central wavelength of the succeeding pulse substantially matches the central wavelength command value. In a step S4, spectral distribution data is integrated with each exposure.