Abstract: In a filter element, at both ends of a cylindrical main filter medium having a space formed on its inner periphery side there are respectively formed a first cap having a communication port for communication of the space with the exterior and a second cap for closing the space, and a pre-filter medium is inserted removably into the space from the communication port. A case comprises a body case for accommodating the filter element removably from an opening thereof in a state in which a gap is formed between the body case and the outer periphery of the main filter medium, and a lid for closing the opening. With the lid attached to the body case, a gas inflow space (flow path) with the communication port positioned therein and a gas outflow space (flow path) with the gap positioned therein are partitioned from each other by a partitioning portion (upper fitting portion).