[en] Purpose: To provide a filter which has a good effect in removal of insoluble material within cooling water in a reactor, has heat resistance and has no effluent therefrom. Constitution: In order to cool non-radioactive or radioactive insoluble material within cooling water in a reactor, a filter is used in which carbon fibers as a filtering material are wound about a cylindrical porous body to form a filtering layer of which opposite ends are retained by stops. The porous body is formed of anticorrosive metal, ceramics, glass, carbon material and the like, and preferably has its diameter from 5 to 200 μ. The carbon fibers include carbonaceous fibers or graphite fibers formed of polyacrylonitrile fibers, fibrous fibers and the like and preferably have their diameter from 0.1 to 100 μ. (Yoshihara, H.)