PURPOSE: To adjust the position of a fin to be attached by fixing a fin position- adjusting fixture on the fin, and attaching the fin on a radiator pipe with the fixture detachably.

CONSTITUTION: An exhaust gas inflow pipe 7 is inserted into a trap main body 2 from the outside of an upper support lid 4, and a radiator pipe 8 is extended to the bottom of the trap main body 2 by inserting it from the outside of the lid 4 through the middle of the main body 2. More than one fin 9 with protrusions is installed on the outer wall of the radiator pipe 8. The intervals of the fins 9 are adjusted to become narrower from the gas inflow side of the upper part of the main body 2 to the lower part. The fins 9 are attached on the radiator pipe 8 detachably. In this way, the contact area of exhaust gas is enlarged to increase efficiently the fixing amount of unreacted gas.