1. A method for assessing irradiation intensity of a laser beam including steps of: irradiating the laser beam onto a reference object on which a change in its state is caused by irradiating the laser beam, and a light quantity of transmitted light of specified illumination light changes in accordance with an irradiation energy density of the laser beam when the illumination light is projected; obtaining an image formed by a change in the light quantity of the transmitted light, occurring when the illumination light is projected on the reference object having been irradiated by the laser beam; and analyzing a condition of the change in the reference object based on the obtained image, wherein an irradiation intensity distribution of the laser beam is assessed based on an analytical result.