PROBLEM TO BE SOLVED: To suppress degradation of durability of a slag melting burner and increase of fuel consumption in a coal gasification furnace. SOLUTION: This slag monitoring device 100 of the coal gasification furnace includes a slag hole camera 11 for observing a slag hole 3 from which molten slag flows out, a water surface camera 12 for observing the slag flowing out from the slag hole 3 and dropping to a water surface 5H of cooling water 5, a drop sound sensor 13 for observing the sound generated when the slag drops to the water surface 5H, and a processing device 20 for determining a slag solidification and attachment part on the basis of an opening area of the slag hole 3 observed by the slag hole camera 11, a dropping line of the slag observed by the water surface camera, and a dropping position of the slag. COPYRIGHT: (C)2011,JPO&INPIT