A plate-like laser medium has a through-hole for providing a flow of a cooling medium. The laser medium unit includes the plurality of laser media. A laser beam amplification device includes a laser medium unit 10, an excitation light source 21 that causes excitation light to enter the laser medium unit 10, a through-hole 16a of a window member as a unit for supplying the cooling medium in a through-hole 14a of the laser medium 14, and a cooling medium flow path F1 arranged around the laser medium unit 10.