An oxygenator that inhibits or prevents bubbles in blood from exiting through a blood outlet includes an oxygenator part which performs gas exchange on blood and a heat exchanging part which performs heat exchange on the blood. The oxygenators part has a housing that is generally in a rectangular parallelepiped form, with a hollow fiber membrane bundle positioned in the housing. The hollow fiber membrane bundle is formed by a multiplicity of hollow fiber membranes adapted to perform gas exchange. Blood flows along a blood passage comprised of gaps between the hollow fiber membranes and contacts the surface of the hollow fiber membranes where gas exchange occurs with gas flowing through the lumens of the hollow fiber membranes. In addition, a filter member is arranged on a downstream side of the hollow fiber membrane bundle so that bubbles present in the blood are caught by the filter member.