A seal structure that is capable of actively controlling the actuation of a movable seal ring, enhancing sealing performance, and enhancing reliability, is provided. A seal structure (41), which partitions a high-pressure portion and a low-pressure portion including spring-back seal rings (51) on the high-pressure side; including ACC seal rings (49) and fins (71) on the low-pressure side, the ACC seal rings (49) having an adjustable movable seal ring (73) which is a movable seal ring (73) that is movable in a substantially radial direction at a portion thereof and which partitions a high-pressure side steam chamber (43) and a medium-pressure side steam chamber (45) with the fins (71) and a circumferential surface (83); and including a pressure equalizing pipe (47) that connects the low-pressure side of the adjustable movable seal ring (73) to a high-pressure outlet portion (25), is provided with an adjusting pipe (55) that connects, on the high-pressure side steam chamber (43) side of the ACC seal rings (49), a high-pressure dummy (39) to the same high-pressure outlet portion (25) as the medium-pressure side steam chamber (45), and an open/close valve (57) in the adjusting pipe (55).