The present invention provides a non-laser-active solid which is connected to a laser-active solid which has a reflective layer system and is cooled by a cooling member. Parallel isothermal planes are created in laser-active solid which are passed through by laser beam at a shallow angle. In this manner, a compact laser beam amplifier system is created which functions without a costly beam shaping optical system for semiconductor laser, and has a good beam quality even at high power.