A seal device, comprising: a pair of sealing units (4, 5) each having a pressing part (42, 52) for sandwiching and pressing an object to provide a seal; and a medium passage that sends a heating medium to at least one of said pair of sealing units (4, 5), said at least one sealing unit (4, 5) including a communication part for the heating medium to pass through for heat exchange between said pressing part (42, 52) and the heating medium, wherein said heating medium is steam, and said medium passage includes a heating medium passage portion for sending a heating medium to said communication part so as to heat said pressing part (42, 52) of said at least one sealing unit (4, 5), a cooling medium passage portion for sending a cooling medium to said communication part so as to cool said pressing part (42, 52) of said at least one sealing unit (4, 5), and a passage switch unit that switches between a state in which said heating medium passage portion and said communication part are connected, and a state in which said cooling medium passage portion and said communication part are connected, characterized in that said medium passage includes a discharge passage portion that is configured to open and close the communication between said communication part and outside of the device so as to discharge the heating medium and the cooling medium from said communication part to the outside of the device; and said discharge passage portion is configured to open said communication part to the outside of the device before said passage switch unit is switched over to a state wherein said heating medium passage portion is connected with said communication part, and closes said communication part from the outside of the device after a predetermined time has passed in the state wherein said heating medium passage portion is connected with said communication part. a pair of sealing units (4, 5) each having a pressing part (42, 52) for sandwiching and pressing an object to provide a seal; and a medium passage that sends a heating medium to at least one of said pair of sealing units (4, 5), said at least one sealing unit (4, 5) including a communication part for the heating medium to pass through for heat exchange between said pressing part (42, 52) and the heating medium, wherein said heating medium is steam, and said medium passage includes a heating medium passage portion for sending a heating medium to said communication part so as to heat said pressing part (42, 52) of said at least one sealing unit (4, 5), a cooling medium passage portion for sending a cooling medium to said communication part so as to cool said pressing part (42, 52) of said at least one sealing unit (4, 5), and a passage switch unit that switches between a state in which said heating medium passage portion and said communication part are connected, and a state in which said cooling medium passage portion and said communication part are connected, characterized in that said medium passage includes a discharge passage portion that is configured to open and close the communication between said communication part and outside of the device so as to discharge the heating medium and the cooling medium from said communication part to the outside of the device; and said discharge passage portion is configured to open said communication part to the outside of the device before said passage switch unit is switched over to a state wherein said heating medium passage portion is connected with said communication part, and closes said communication part from the outside of the device after a predetermined time has passed in the state wherein said heating medium passage portion is connected with said communication part.