To enhance durability and reduce time for the stabilization of a furnace air by interposing a plurality of metal-made sheets or foils into insulation structure with a specified spacing in in-furnace insulation structure where fibrous insulating materials are laminated.

An insulation material 2 is mounted inside a furnace of a furnace shell 1 with metal fittings 3a and 3b. The insulation material 2 is an insulation board 2c which comprises a ceramic fiber-made heat resistant board 2a, a Kaowool blanket 2b and a ceramic fiber where a metal foil 7a or 7b is interposed respectively between the heat resistant board 2c and the blanket 2b and between the blanket 2b and the heat resistant board 2a. Mounting pins 4a and 4b are welded with the side surface of the furnace shell 1 on the in-furnace side and a through hole-formed insulation board 2c and the metal foil 7a are inserted and fixed with a presser metal fitting 5 while the metal foil 7b and the heat resistant board 2a are inserted and fixed with the presser metal fitting 5 and a nut 6.