A gas sample vessel for a gas analyzer comprising a housing enclosing a cylindrical resonator cavity. Two mirrors which limit the resonator cavity are fastened to the housing for coupling in and feeding back the measurement light proceeding from an illumination source. The mirrors are curved concavely at their surfaces facing the housing and the housing is curved convexly at its surfaces facing the mirrors with approximately the same radius of curvature. The centers of curvature lie in the center axis of the resonator cavity and the mirror surfaces of the mirrors are portions of the concavely curved surfaces facing the housing.