The invention aims at provision of an optical scanner in which an optics required for equalizing an optical magnification in a scanning range is produced with high accuracy, so that intervals among a plurality of scanning lines on a to-be-scanned surface are even in the scanning range. The optical system includes an optical axis, a spherical concave reflection mirror; a lens group with positive refracting power arranged adjacent the mirror with an air space therebetween. Optionally, this heating step may be performed prior to any substantial removal of water-soluble minerals from the organic-rich rock formation. The fixed leg has a moveable shank therein, and a notch is formed in an inner side of the moveable shank.