A ring segment includes segment bodies arranged along a circumferential direction; a main cavity; first cooling channels inside the segment body to extend along an axial direction of a rotor and arrayed in the circumferential direction, and whose ends communicate with the main cavity on an upstream side thereof; a second cooling channel inside the segment body on an upstream side in a rotation direction of the rotor to extend along the axial direction, and whose first end communicates with the main cavity on the upstream side thereof; and third cooling channels to extend along the circumferential direction, in a predetermined region forming a part of a lateral end of the segment body on the upstream side and stretching from an end of the segment body on a downstream side in the combustion gas flow direction toward the upstream side, and whose first ends communicate with the second cooling channel.