A ring segment of a gas turbine, the ring segment being formed as a plurality of segment bodies is arranged annularly along a circumferential direction (Dc) around an axial direction (Da) which is parallel to a rotor in a mounted state in the gas turbine and which corresponds to a combustion flow direction in operation which defines an upstream side in the axial direction (Da) and a downstream side in the axial direction (Da), the circumferential direction (Dc) corresponding to a rotation direction of the rotor in operation which defines an upstream side in the circumferential direction (Dc) and a downstream side in the circumferential direction (Dc), the segment body comprising: a main cavity that is provided on an outer side of the segment body in a radial direction (Dr) of the ring segment and configured to receive cooling air from outside, the main cavity being partitioned by an impingement plate fixed to a recess in an outer surface of the segment body; a plurality of first cooling channels which is provided inside the segment body so as to extend along the axial direction (Da) and so as to be arrayed in the circumferential direction (Dc), and of which first ends communicate with the main cavity on the upstream side thereof in the axial direction (Da) while second ends open toward the downstream side of the segment body in the axial direction (Da); a second cooling channel which is disposed inside the segment body on the upstream side thereof in the circumferential direction (Dc) so as to extend along the axial direction (Da), and of which a first end communicates with the main cavity at an opening on the upstream side of the main cavity in the axial direction (Da); and a plurality of third cooling channels which is provided inside the segment body so as to extend along the circumferential direction (Dc), of which first ends communicate with the second cooling channel while second ends open at a lateral end on the upstream side in the circumferential direction (Dc); characterised in that the second cooling channel is provided at a position of the segment body further on an inner side in the radial direction (Dr) than the main cavity; and in that the plurality of third cooling channels is provided only in a predetermined region (Xl) that is disposed along a part of the overall length in the axial direction (Da) of the lateral end of the segment body on the upstream side in the circumferential direction (Dc) between an intermediate position in the axial direction (Da) and an end of the segment body on the downstream side in the axial direction (Da). a main cavity that is provided on an outer side of the segment body in a radial direction (Dr) of the ring segment and configured to receive cooling air from outside, the main cavity being partitioned by an impingement plate fixed to a recess in an outer surface of the segment body; a plurality of first cooling channels which is provided inside the segment body so as to extend along the axial direction (Da) and so as to be arrayed in the circumferential direction (Dc), and of which first ends communicate with the main cavity on the upstream side thereof in the axial direction (Da) while second ends open toward the downstream side of the segment body in the axial direction (Da); a second cooling channel which is disposed inside the segment body on the upstream side thereof in the circumferential direction (Dc) so as to extend along the axial direction (Da), and of which a first end communicates with the main cavity at an opening on the upstream side of the main cavity in the axial direction (Da); and a plurality of third cooling channels which is provided inside the segment body so as to extend along the circumferential direction (Dc), of which first ends communicate with the second cooling channel while second ends open at a lateral end on the upstream side in the circumferential direction (Dc); characterised in that the second cooling channel is provided at a position of the segment body further on an inner side in the radial direction (Dr) than the main cavity; and in that the plurality of third cooling channels is provided only in a predetermined region (Xl) that is disposed along a part of the overall length in the axial direction (Da) of the lateral end of the segment body on the upstream side in the circumferential direction (Dc) between an intermediate position in the axial direction (Da) and an end of the segment body on the downstream side in the axial direction (Da).