A seal structure for a rotary machine, configured to partition a high-pressure portion and a low-pressure portion, comprising: at least one stage of high-pressure side seal rings provided on a high-pressure side of the seal structure, the stage of high-pressure side seal rings being configured to be held in a stationary portion of the rotary machine, so as to oppose a circumferential surface of a rotating member of the rotary machine; at least one stage of low-pressure side seal rings provided on a low-pressure side of the seal structure, the stage of low-pressure side seal rings being -configured to be held in the stationary portion, so as to oppose the circumferential surface of the rotating member and fins, which protrude in a ring-like manner, being configured to be provided in at least one of the low-pressure side seal rings and the rotating member, wherein the low-pressure side seal rings are configured to partition the high-pressure portion and the low-pressure portion with the fins and an opposing surface opposing the fins, the low-pressure side seal rings have adjustable movable seal rings being movable in a substantially radial direction at least at a portion thereof in the circumferential direction and which are biased outward by an elastic piece, the seal structure is configured so that, when installed in the rotary machine, a gap is formed between an inner circumferential surface of the stationary portion and an outer circumferential surface of the low-pressure side seal rings, and the low-pressure side seal rings include a communication hole that provides communication between the high-pressure side of the low-pressure side seal rings and the gap. the seal structure being characterized by further comprising: a connecting flow path configured to continuously provide communication between a single pressure source and a low-pressure side of the low-pressure side seal rings, an adjusting flow path configured to provide communication between a high-pressure side of the low-pressure side seal rings and the single pressure source; and an open/close valve that is provided in the adjusting flow path to open and close the adjusting flow path, the open/close valve being configured to provide communication between the high-pressure side of the low-pressure side seal rings and the single pressure source when the open/close valve is opened at least one stage of high-pressure side seal rings provided on a high-pressure side of the seal structure, the stage of high-pressure side seal rings being configured to be held in a stationary portion of the rotary machine, so as to oppose a circumferential surface of a rotating member of the rotary machine; at least one stage of low-pressure side seal rings provided on a low-pressure side of the seal structure, the stage of low-pressure side seal rings being -configured to be held in the stationary portion, so as to oppose the circumferential surface of the rotating member and fins, which protrude in a ring-like manner, being configured to be provided in at least one of the low-pressure side seal rings and the rotating member, wherein the low-pressure side seal rings are configured to partition the high-pressure portion and the low-pressure portion with the fins and an opposing surface opposing the fins, the low-pressure side seal rings have adjustable movable seal rings being movable in a substantially radial direction at least at a portion thereof in the circumferential direction and which are biased outward by an elastic piece, the seal structure is configured so that, when installed in the rotary machine, a gap is formed between an inner circumferential surface of the stationary portion and an outer circumferential surface of the low-pressure side seal rings, and the low-pressure side seal rings include a communication hole that provides communication between the high-pressure side of the low-pressure side seal rings and the gap. the seal structure being characterized by further comprising: a connecting flow path configured to continuously provide communication between a single pressure source and a low-pressure side of the low-pressure side seal rings, an adjusting flow path configured to provide communication between a high-pressure side of the low-pressure side seal rings and the single pressure source; and an open/close valve that is provided in the adjusting flow path to open and close the adjusting flow path, the open/close valve being configured to provide communication between the high-pressure side of the low-pressure side seal rings and the single pressure source when the open/close valve is opened a connecting flow path configured to continuously provide communication between a single pressure source and a low-pressure side of the low-pressure side seal rings, an adjusting flow path configured to provide communication between a high-pressure side of the low-pressure side seal rings and the single pressure source; and an open/close valve that is provided in the adjusting flow path to open and close the adjusting flow path, the open/close valve being configured to provide communication between the high-pressure side of the low-pressure side seal rings and the single pressure source when the open/close valve is opened