PURPOSE: To prevent the surface of an acoustic sensor from being directly exposed to a furnace gas, and to prevent the acoustic sensor from being raised to high temperatures by providing the acoustic sensor in a cylindrical body, feeding the sealing air, and cooling the cylindrical body.

CONSTITUTION: A sound transmission pipe 12 is constituted of two pipes, namely, a cylindrical body 13 and an outer cylinder 14. An acoustic sensor 19 is provided in the cylindrical body 13. An inlet 25 for sealing air is opened at the outer part of the cylindrical body 13 to feed the air to the cylindrical body 13. An outer cylinder 14 is provided outside the cylindrical body 13. A cooling liquid is supplied between the cylindrical body 13 and the outer cylinder 14 to cool the cylindrical body 13 and the acoustic sensor 19.