An optical axis or a diameter of light of even a light beam having a non-uniform intensity distribution is stably obtained. An outer shape of a range of a light intensity having a specified ratio with respect to a total light intensity on a cross-section of a light beam is obtained. An approximate circle is obtained from this outer shape. For example, the approximate circle is calculated by using the least-squares method. An optical axis and/or a diameter of light of the light beam is obtained on the basis of this approximate circle.