PROBLEM TO BE SOLVED: To keep oil-adsorption function stable without spoiling the function and hindering oil-passing inside caused by falling a large amount of bark having a length of specified value.
SOLUTION: Bark 2, from several mm to several cm in length, is put in an outer cover 1 formed of a cylindrical net. A partition wall 5, whose material is the same as that of the outer cover 1, is equipped at proper intervals so that the bark 2 does not move inside. Rope 4 used for spreading or recovering work is set on the center part of or the outer circuit of the outer cover 1. The material of the outer cover 1 allows an inexpensive net made of polyethylene, a used fishing-net or the like, if it is of such strength as not to break and so light as not to sink its body during working, however, in order not to lose the advantage of easy burning treatment of the after-using bark, a natural material such as cotton or the like having the same advantage is preferably used.