Provided is a light reduction mechanism which includes a plate-type first beam splitter and a plate-type second beam splitter, in which in a case where the X axis and the Y axis are arbitrary orthogonal coordinate axes in the plane perpendicular to the optical axis of laser beam irradiation optical unit whose origin is in the optical axis, the first beam splitter is arranged with the X axis as a rotation axis and inclined at an angle α in the range from 30° to 60° inclusive with respect to the plane perpendicular to the optical axis, and the second beam splitter is arranged with the X′ axis, which is parallel to the X axis and passes through the optical axis, as a rotation axis, and inclined at an angle −α with respect to the plane perpendicular to the optical axis.