A balance ratio for a stationary seal ring is designed to allow a seal face to be opened when, with a rotary seal ring at standstill, steam of a predetermined pressure is supplied to an endo-fluid region while a sealing-fluid region is brought into an exhausting state. On the other hand, an O-ring constituting a secondary seal portion is retained by a groove formed in a flange portion of a seal case, thereby preventing a force on the O-ring from affecting the stationary seal ring even when the steam is supplied to the endo-fluid region.