The narrow-band laser device may comprise: a laser resonator containing an optical element for narrowing the band of a spectral line width; a spectrometer detecting spectral intensity distribution fora plurality of pulses contained in a pulsed laser light outputted from the laser resonator; a spectral wave form generation unit generating a spectral wave form resulting from the addition of the spectral intensity distributions of a plurality of pulses; a device function storage unit for storing the device functions of the spectrometer; a wavelength frequency function generation unit generatinga wavelength frequency function representing the frequency distribution of the central wavelength for the plurality of pulses; and a deconvolution processing unit using the device function and the wavelength frequency function to carry out deconvolution processing on the spectral wave form.