To prevent a great vibration from being transmitted to a heat-treated material placed on a hearth, enable a positive heat treatment of the heat-treated material to be positively carried out and at the same time enable a sealing characteristic of a heat treatment space within a main body of the furnace to be kept for a long period of time.

A heat-sealing mechanism 15 is comprised of a ring-shaped container formed continuously along a moving direction of a hearth 13 and having silicone oil charged in it and a sealing plate formed continuously along the moving direction of the hearth 13 and inserted into silicone oil with the container. Then, the container is fixed to the hearth 13 in such a way that an opening 14a of a clearance 14 may be closed by the container and the sealing plate. The sealing plate is fixed to a main body 12 of the furnace. In addition, the sealing plate is provided with a silicone rubber so as to cover the surface of silicone oil in the container. Heat within the heat treatment space 11 is prevented from being lost through the main body 12 of the furnace having a ring-like heat treatment space 11 within it, the hearth 13 having a heat treated material W installed thereon and rotationally moved in the heat treatment space 11, and a clearance 14 formed between the main body 12 of the furnace and the hearth 13.