A cooling system of a ring segment for a gas turbine, which includes first cooling passages disposed in an axial direction of the rotating shaft of a main body of the segment body, second cooling passages disposed at one end portion in a direction approximately perpendicular to the first cooling passages, and blowing a cooling air toward the end portion of a neighboring segment body, and third cooling passages connecting a first cavity, which is disposed approximately perpendicular to the axial direction of the rotating shaft at the upstream-end portion, with a cooling space, which is surrounded by the segment body and a collision plate having a plurality of small holes. The first cooling passages include cooling passages of a first region and a second region having a smaller passage cross-sectional area than the first-region cooling passages or a greater arrangement pitch than the first-region cooling passages.