PURPOSE: To allow the height of a fence to be simply altered in one fence by providing a holder having a holding part to be inserted with the leg of the fence to be held, and a mounting protrusion so protruding on the outer periphery or the part and inserted and rotated to a locking hole to fix the part to a returning grill.

CONSTITUTION: One fence 8 having a suitable height is provided, and has a rodlike leg 8 protruding downward. A holder 95 has a cylindrical holding part 96 with a bottom in which the leg 8 is freely detachable and a mounting protrusion 97 protruding on the outer periphery of the part 96 and having a narrow root width formed in a neck part. On the other hand, a plurality of upper and lower rectangular locking holes 35 in which the protrusion 97 can be inserted are formed at the riser of a returning grill 31, an insertion hole 35 of the size in which a hand can be inserted is opened at the upper surface of the hole 35. A cover 37 having a hole 37a in which the leg 81 of the fence can be passed is provided at the hole 36.