The present disclosure relates to a filter. According to the present disclosure, there is provided a filter including: an upper body whose interior is filled with a filter medium; a lower body located under the upper body in such a manner as to be open on top and bottom thereof; and an escape prevention member located inside the lower body to allow sludge and a fluid to be introduced into the lower body and to prevent the sludge from escaping from the lower body, wherein the escape prevention member may include: a support plate fixed to the inside of the lower body and having a through hole formed at the center thereof; a center pipe fixed to top of the support plate and having a lower hole formed on the bottom thereof in such a manner as to communicate with the through hole and at least one or more connection holes formed on one side thereof; and a backflow prevention ball movable inside the center pipe in such a manner as to selectively open and close the lower hole to prevent the fluid from flowing back.