A gas turbine rotor, more particularly a gas turbine rotor blade, which is manufactured so as to be longer, larger, and thin walled and is provided on a rear side of a gas turbine blade array. The gas turbine rotor blade is cooled with cooling air flowing interiorly thereof. A cavity is provided to facilitate the flow of cooling air. The cavity is formed inside a rotor root and inside a hub unit disposed adjacent to a rotor profile unit. Projections are provided inside of the cavity so that the projections, which protrude from an inner wall of the cavity, project into the cooling air flow. Consequently, cooling efficiency of the rotor blade can be improved, and the strength of the portions forming the cavity can be significantly increased. The device also allows the core for cooling to be manufactured and easily set.