To provide a seal structure with excellent workability enabling the easy and rapid installation and sealing of a measuring instrument such as a thermocouple.

This seal structure comprises a tubular connection member 30 in which one end part is inserted into a first through hole 11 passed through a first partition wall 10 and the other end part is inserted into a second through hole 21 passed through a second partition wall 20 opposed to the first partition wall 10, the bar-like measuring instrument 40 inserted through a connection member 30 from the inside of the first partition wall 10 to the inside of the second partition wall 20, a first seal member S1 disposed between the first through hole 11 and the connection member 30 and sealing between the inside and outside of the first partition wall 10, a second seal member S2 disposed between the second through hole 21 and the connection member 30 and sealing between the inside and outside of the second partition wall 20, and a third seal member S3 disposed between the measuring instrument 40 and the connection member 30, the first partition wall 10, or the first partition wall 20 and sealing between the inside of the first partition wall 10 and the inside of the second partition wall 20. The measuring instrument such as the thermocouple can be very easily and rapidly installed by allowing the inside of the first partition wall to communicate with the inside of the second partition wall to seal the inside and outside thereof, and since the inside of the first partition wall and the inside of the second partition wall are sealed, the seal structure with excellent workability can be realized.