In a slot array structure, electromagnetic wave emission which is uniform as a whole over the length of a laser tube is realized to allow uniform laser emission with minimum energy loss. Slots (10) are formed at a predetermined pitch in a long end face (H plane) of each waveguide (1) along a central line (m) in the longitudinal direction of the H plane to be alternately located on the left and right sides of the central line (m) and spaced apart from the central line (m) by a distance (d).