A seal structure for a gas turbine rotor blade arranged at a rotor shaft in a circumferential direction and extending in a radial direction of the rotor shaft, the seal structure comprising: a plurality of rotor blades each including a blade body, a shank including a platform and a root portion, and a fin projecting forward of the blade body from the shank; and a wedge seal member and a spline seal member positioned between the respective rotor blades adjacent to each other, the shank including a first side surface, a second side surface, and a bottom surface located inward in the radial direction from the fin, the first side surface including a recess extending in a direction from a front side to a rear side of the blade body to house the wedge seal member movably in the radial direction and the circumferential direction, and a first slot extending in a direction from the platform to the root portion, the second side surface including a second slot provided at a position corresponding to the first slot provided in the shank of the rotor blade adjacent to the corresponding shank in which the second slot is provided, the recess including an inclined surface extending straight inward in the radial direction and away from the side surface, the wedge seal member including a wedge portion including a first wedge surface to be opposed to the inclined surface and a second wedge surface to be opposed to the second side surface of the adjacent rotor blade to form a top portion of a wedge together with the first wedge surface, and a weight portion integrated with the wedge portion and positioned inward in the radial direction from the wedge portion in the recess, characterized in that the first slot and the second slot each opens on the bottom surface of respectively the first side surface and the second side surface to serve as the insertion opening for the spline seal member; and that an inner surface of each of the first and second slots (40, 43) includes a receiving portion projecting forward as a part of an edge of the insertion opening to receive the spline seal member. a plurality of rotor blades each including a blade body, a shank including a platform and a root portion, and a fin projecting forward of the blade body from the shank; and a wedge seal member and a spline seal member positioned between the respective rotor blades adjacent to each other, the shank including a first side surface, a second side surface, and a bottom surface located inward in the radial direction from the fin, the first side surface including a recess extending in a direction from a front side to a rear side of the blade body to house the wedge seal member movably in the radial direction and the circumferential direction, and a first slot extending in a direction from the platform to the root portion, the second side surface including a second slot provided at a position corresponding to the first slot provided in the shank of the rotor blade adjacent to the corresponding shank in which the second slot is provided, the recess including an inclined surface extending straight inward in the radial direction and away from the side surface, the wedge seal member including a wedge portion including a first wedge surface to be opposed to the inclined surface and a second wedge surface to be opposed to the second side surface of the adjacent rotor blade to form a top portion of a wedge together with the first wedge surface, and a weight portion integrated with the wedge portion and positioned inward in the radial direction from the wedge portion in the recess, characterized in that the first slot and the second slot each opens on the bottom surface of respectively the first side surface and the second side surface to serve as the insertion opening for the spline seal member; and that an inner surface of each of the first and second slots (40, 43) includes a receiving portion projecting forward as a part of an edge of the insertion opening to receive the spline seal member.