To enable a practical molten state in an ash melting furnace to be clearly monitored by a method wherein an infrared-ray camera is fixed in a through-pass hole formed in a furnace body and a melting state in the furnace is remotely monitored through the infrared-ray camera.

An infrared ray camera 2 is fixed to a through-pass hole arranged at a side wall of a main body 1 of a furnace. A lens tube 4 is fixed to a camera case 2a enclosing the infrared ray camera 2, a jacket 5 fixed to the lens tube 4 is fixed to a side wall of the main body 1 of the furnace. A filter 3 is fixed to a front side of the infrared ray camera 2 within the camera case 2a, cylindrical tube 4 having its extremity end closed is fixed to the front side of the filter 3, and a plurality of lenses 4a are stored in the lens tube 4. A cylindrical jacket 5 having its extremity end being released with a clearance 5a left between it and the lens tube 4 is fixed to and outer circumference of the lens tube 4, and the jacket 5 is inserted into an fitted to the through-pass hole at the side wall of the main body 1 of the furnace. Then, the infrared ray camera 2 is connected to a monitor 6.