An injection locking type or MOPA type of gas laser device which requires only a small installation area and allows easy maintenance. For this purpose, the laser device includes a seed laser unit (11) for exciting a laser gas inside a laser chamber (12) and oscillating seed laser light (21) and an amplifier (111) for exciting a laser gas inside an amplifying chamber (112) and amplifying pulse energy of the seed laser light (21), and allows the laser chamber (12) and the amplifying chamber (112) to be evacuated to the same side with respect to an optical axis of the seed laser light (21).