Laser medium, laser medium unit, and laser beam amplification device

A laser medium unit including laser media, each of the laser media is a plate-like laser medium comprising a through-hole for making a cooling medium pass through the through-hole, wherein excitation light enters the laser medium from an outer peripheral surface of the laser medium, wherein

• a thickness of the plate-like laser medium is from 1 mm to 20 mm, inclusive,
• the laser media are stacked in the thickness direction thereof,
• a ring-shaped sealing material is interposed between adjacent laser media;
• and
• the sealing material receives pressure in the stacking direction of the laser media.