PURPOSE: To eliminate the need to cut and sample part of a metal-sprayed furnace wall tube by fitting a semicylindrical test-piece made of substantially the same material as the material of the furnace wall tube of a boiler to part of the furnace wall tube and spraying metal, and detaching the test-piece from the furnace wall tube and confirming whether or not the metal-spraying process is carried out normally.

CONSTITUTION: The test-piece 4 is fitted detachably to part of the furnace wall tube 3 to be metal-sprayed by a coupling means 5 such as a magnet or both-sided adhesive tape. The test-piece 4 is made semicylindrical because the furnace wall tube 3 is tubular and its metal-sprayed film varies in property according to a circumferential position. The test-piece 4 is made of exactly or nearly the same material with the furnace wall tube 3 substantially and a little bit larger than the external diameter of the furnace wall tube so that both metal-sprayed films have no different in property. The test-piece 4 is fitted to the furnace wall tube 3, grit blasting is carried out, and metal spraying is performed from it; and the test-piece 4 is detached from the furnace wall tube 3 and it is confirmed whether the metal-spraying process is performed normally or not.