Disclosed herein is a filtering apparatus for treating ballast water. The apparatus is constructed so that a housing cover plate and an exhaust unit are removably coupled to each other at an upper position of a housing. The exhaust unit includes a base flange on a portion thereof coupled to the housing cover plate and the base flange includes a projecting step that projects to be inserted into a core through hole formed in the housing cover plate. A core serving as a discharge passage of foreign substances includes a shaft coupling unit on a portion thereof coupled to the driving shaft, and is coupled at the shaft coupling unit to the driving shaft via a coupling structure. The outer diameters of the coupling structure and the shaft coupling unit are formed to be smaller than the outer diameter of the core.