A filtering device for treating ballast water and more particularly, a multicage-type device for filtering ballast water to prevent back pressure, wherein the multicage-type device for filtering ballast water includes a back pressure prevention tank on a backwash line for backwashing and discharging foreign substances that are inside a filter of the device for filtering, so that backwash water and the foreign substances in the backwash line are preliminarily stored in the back pressure prevention tank thereby preventing an increase of back pressure and preventing an increase of the back pressure on the backwash line by always maintaining a level of water inside the back pressure prevention tank at lower than an inlet port that is connected to the backwash line.