A cooling system of ring segment is provided with: a collision plate that has a plurality of small holes; a cooling space that is enclosed by the collision plate and a main body of the segment body; a first cavity that arranged is 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 a rotating shaft; a first cooling passage that communicates from the cooling space to the first cavity; and a second cooling passage that communicates from the first cavity to a fire combustion gas d gas space in the downstream end portion of the segment body in the flow direction of the combustion gas.