Axial-flow turbine and power plant including the same

An axial-flow turbine (10) according to an embodiment includes a plurality of nozzle structures (20) and a plurality of blade structures (40). At least one nozzle structure includes an outer ring diaphragm (21) and an inner ring diaphragm (22). The outer ring diaphragm and the inner ring diaphragm form an annular opening portion which extends in a circumferential direction therebetween. A nozzle (25) is provided in a portion of a region of the annular opening portion in the circumferential direction, and a closing part (30) is provided in another portion of the region of the annular opening portion in the circumferential direction. The closing part closes this other portion of the region to prevent a working fluid from flowing into this other portion of the region. A closing part medium passage (33) is provided in the closing part and is configured to flow a cooling medium which cools the closing part.