Provided is a beam-quality evaluation method that makes it possible to evaluate, irrespective of the beam diameter of a beam being evaluated, the beam diameter, wavefront aberration, divergence angle, and light intensity, inexpensively and with high precision. The present invention is characterized by including: a step for calculating a wavefront aberration by comparing a light intensity distribution of a diffraction image with individual light intensity distributions generated by setting the beam diameter error to 0 and using individual wavefront aberrations for wavelengths greater than or equal to 0 as a parameter; a step for calculating a divergence angle by using the calculated wavefront aberration on the basis of individual light intensity distributions of diffraction images formed at individual distances from a light source to a diffractive optical element; a step for calculating the beam diameter immediately after radiation by comparing a light intensity distribution generated by using the design value of the beam diameter as well as the calculated wavefront aberration and divergence angle with the light intensity distribution of the diffraction image; and a step for calculating light intensity on the basis of a power conversion efficiency of the diffractive optical element and the light intensity of diffracted light, calculated from the light intensity distribution of the diffraction image.
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