To provide a slab type gas laser generating a high quality laser beam having an intensity distribution approximate to Gaussian distribution on the cross-section of an outputted laser beam.

Two tubular electrodes 11, 12 having different diameters are arranged concentrically through a spacer 13 to form a tubular straight slab 1 filled with a laser medium between two tubular electrodes 11, 12. A ring-like trick mirror M1 is disposed at one end part of the straight slab 1, an output mirror M2 passing one part of a light beam and reflecting the other part of the light beam is disposed in the center at one end part of the straight slab 1, and a double action mirror M3 is disposed at the other end part of the straight slab 1. A relation xm≤1.1x0 is set between the radius Xm (distance from the central axis) of the trick mirror M1 at the center of curvature thereof and the radius x0 (distance from the central axis) of the trick mirror M1 at the center of.