PURPOSE: To quickly and cleanly thermally decompose and remove attached polymer, etc., by covering the opening of a fine hole of material to be treated with a specific filter and treating the covered opening in a fluid layer furnace in a state where the filter is situated on the lower side.

CONSTITUTION: At least either one end face in which a fine hole 3 of material to be treated, e.g. spinneret 1 is opened is shielded with a filter 2 through which a fluid medium of furnace bed is not passed, but air gas can be passed, and the filter 2 is dipped into a fluid layer in a fluid layer furnace so that the filter is situated on the lower side. Thereby a polymer, etc., attached to the fine hole 3, etc., is thermally decomposed and removed.