An oxygenator includes: a housing; a hollow fiber membrane bundle stored in the housing and having multiple integrated hollow fiber membranes with a gas exchange function; a gas inlet portion and a gas outlet portion provided on the upstream and downstream of gas passages in lumens of the hollow fiber membranes, respectively; a blood inlet portion and a blood outlet portion provided on the upstream and downstream of blood passages outside the hollow fiber membranes, respectively; a first filter member provided on the hollow fiber membrane bundle in contact with a blood outlet portion side surface so as to cover substantially the entire surface and has a function to catch bubbles in blood; and a second filter member that is separated from the first filter member, positioned between the first filter member and the blood outlet portion, and has a function to catch bubbles in blood.