A gas turbine first-stage stator blade sealing structure for preventing the sealing air from leaking from the inside even if the blade falls or inclines due to thermal stress. A rear flange (12) of a first-stage stator blade (31) is formed in an arcuate shape having a straight rib (10). A stator blade support ring (13) contacts the straight rib (10) to form a sealing face. When the rear flange (12) and the stator blade support ring (13) fall or incline relative to each other due to thermal stress during operation, the upper end or lower end of the rib (10) contacts linearly with the face of the stator blade support ring (13) so that no clearance is established to inhibit leakage of the sealing air. In the prior art, where the horizontal rib (10) is absent, an arcuate clearance is established between the stator blade support ring (13) and the rear flange (12) as the support ring (13) inclines, and thus sealing air is allowed to leak from the inside to the gas passages.