An oxygenator includes a housing; a hollow fiber membrane layer that is stored in the housing and has multiple integrated hollow fiber membranes with a gas exchange function; a gas inlet portion and a gas outlet portion that are provided on the upstream and downstream of gas passages in lumens of the hollow fiber membranes, respectively; and a blood inlet portion and a blood outlet portion that are provided on the upstream and downstream of blood passages outsides of the hollow fiber membranes, respectively. The hollow fiber membranes in the hollow fiber membrane layer are fixed relative to each other at one end portion and the other end portion thereof. Conditions 30≦ε60 and OD≦4.5×ε are met where ε [μm] is an average separation distance between the adjacent hollow fiber membranes at a fixed portion of the hollow fiber membrane layer and OD [μm] is the outer diameter of the hollow fiber membrane.