Disclosed is a laser medium unit that includes a laser medium and a holding body. The laser medium has a pair of end surfaces. The holding body surrounds the laser medium when viewed form a direction intersecting with the pair of end surfaces and holds the laser medium. The holding body includes a deformation allowing portion that extends from the inside to the outside of the holding body when viewed from the direction intersecting with the pair of end surfaces. The laser medium and the holding body are in contact with each other. A contact region of the holding body with the laser medium has a width in the direction intersecting with the pair of end surfaces and extends along a side surface of the laser medium when viewed from the direction intersecting with the pair of end surfaces.