PURPOSE: To prevent the sticking of dross into a duct and obviate the need for wiping away the dross with cloth, etc., by inserting the specific cutting tool into the duct prior to cutting and boring, and removing this cutting jig after the cutting and boring.

CONSTITUTION: The duct 2 is cut and bored by a cutting device such as plasma cutter. The sticking of the dross produced at this time into the duct is prevented. The cutting jig 10 for preventing the dross sticking is, thereupon formed by equally or randomly combining plural plate materials 11 radially from the center so as to approximately meet the bore of the duct 2. The cutting jig 10 is inserted into the duct at the time of boring and cutting. The cutting jig 10 is removed from the inside of the duct 2 to prevent the sticking of the dross produced at the time of cutting and boring in the duct 2 after the completion of the boring and cutting. As a result, the need for removing a cut piece from the inside of the duct is obviated.