A gas turbine ring segment for a gas turbine. The gas turbine includes a rotor rotating about an axis, a housing containing the rotor to be rotatable, and a stationary wing ring disposed on an inner circumferential portion of the housing to be annular about the axis. A plurality of the gas turbine ring segments is disposed on the gas turbine to be dividable in a circumferential direction. Each of the cooling holes includes a first cooling hole and a second cooling hole having different diameters, the first cooling hole and the second cooling hole being connected serially to each other. This structure controls the flow rate of refrigerant flowing through the cooling holes while maximizing heat transfer efficiency.