A cooling system of a ring segment that is formed from a plurality of segment bodies that is arranged in the circumferential direction of a rotating shaft of a gas turbine to form a ring shape, and that is for cooling a ring segment of the gas turbine that is arranged in a casing so that the inner peripheral surface of each segment body is kept at a fixed distance from the tip of turbine blades, wherein the segment body comprises: a main body and side portions (18,19) that are provided along the axial direction of the rotating shaft; a collision plate that has a small hole that is arranged to cause cooling air that is supplied, in operation, from outside of the casing to be blown out and perform impingement cooling of the main body of the segment body; a cooling space that is enclosed by the collision plate and the main body of the segment body; a first cavity (20;31) that is arranged in the axial direction of the rotating shaft along at least one side end portion (18,19) so as to receive from the cooling space the cooling air after the impingement cooling; first cooling passages, of which one end communicates with the first cavity (20;31), and the other end is arranged to blow out the cooling air from an opening that is arranged in the side end portion (18,19) into combustion gas; a second cavity (23;30) that is arranged at the upstream end portion of the segment body in the flow direction of the combustion gas so as to be perpendicular to the axial direction of the rotating shaft; a second cooling passage that is provided in the axial direction of the rotating shaft and communicates from the cooling space to the second cavity (23;30); and a third cooling passage that is provided in the axial direction of the rotating shaft and opens from the second cavity (23;30) to the combustion gas in the downstream end portion of the segment body, wherein the openings of the first cooling passages are arranged so that the arrangement pitch of the openings becomes smaller or the opening area of the openings becomes larger on the upstream in the flow direction of the combustion gas than the openings on the downstream, and are arranged so that the arrangement pitch of the openings becomes larger or the opening area of the openings becomes smaller on the downstream in the flow direction of the combustion gas than the openings on the upstream. a main body and side portions (18,19) that are provided along the axial direction of the rotating shaft; a collision plate that has a small hole that is arranged to cause cooling air that is supplied, in operation, from outside of the casing to be blown out and perform impingement cooling of the main body of the segment body; a cooling space that is enclosed by the collision plate and the main body of the segment body; a first cavity (20;31) that is arranged in the axial direction of the rotating shaft along at least one side end portion (18,19) so as to receive from the cooling space the cooling air after the impingement cooling; first cooling passages, of which one end communicates with the first cavity (20;31), and the other end is arranged to blow out the cooling air from an opening that is arranged in the side end portion (18,19) into combustion gas; a second cavity (23;30) that is arranged at the upstream end portion of the segment body in the flow direction of the combustion gas so as to be perpendicular to the axial direction of the rotating shaft; a second cooling passage that is provided in the axial direction of the rotating shaft and communicates from the cooling space to the second cavity (23;30); and a third cooling passage that is provided in the axial direction of the rotating shaft and opens from the second cavity (23;30) to the combustion gas in the downstream end portion of the segment body,