An excimer laser apparatus according to the present disclosure includes an etalon spectrometer configured to measure a fringe waveform of a laser beam; and a controller configured to obtain area of a first ratio in a spectral space obtained based on a result of the measurement by the etalon spectrometer, calculate a first spectral line width of the laser beam based on the obtained area of the first ratio, and calibrate a first spectral line width based on a correlation function representing correlation between the first spectral line width and a second spectral line width of the laser beam measured by a reference meter.