To seal a shaft when a rotary shaft 2 is rotated at a high speed as well as when it is stopped in a shaft seal device using an inflatable ring.

When an inflatable ring 12 is inflated to the inner peripheral side by pressurized air, the bellows-like body 131 of a seal ring 13 locked by a detent flange section 132 is brought into close contact with the inflatable ring 12 and a rotary shaft 2 at a proper seal surface pressure. The seal ring 13 is locked and is made of a low-friction sliding material, thus the sliding heat and the torsional torque applied to the inflatable ring 12 are suppressed small even when the rotary shaft 2 is rotated. The reduction of the seal bearing pressure caused by the aging abrasion of the inner peripheral face of the bellow- like body 131 is compensated by an increase of the pressing force of the inflatable ring 12 by the pressurized air.