PURPOSE: To reduce the area of observation window by putting the scanning origin by a detector of infrared rays incident through the window in the position of the window.

CONSTITUTION: The infrared rays from the in-furnace high temperature objects, etc. incident to a see-through window 1 such as of silicon or the like provided to a furnace wall 2 is focused by the condensing mirror 5, vibrating mirror 6, lens 7, etc. of a box body 8 and is detected with a detector 4. The incident infrared rays are scanned in the two orthogonal directions by the reciprocating motion within a fixed angle range centering at the central part O of the vibrating mirror 6 turning about the axis P and the window 1 of the box body 8. Making this window 1 the scanning origin obviates the decrease in the field of observation despite the reduction in the size of the window 1 in the scanning direction of the box body 8 and enables the window 1 to be the one of a smaller area. As a result, the thermography apparatus may be constituted with the wondow of a small area and high strength and the small size cleaning device.