An oxygenator includes: a housing; a hollow fiber membrane layer that is stored in the housing and has multiple integrated hollow fiber membranes 31 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 31, 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 31, respectively. The hollow fiber membranes 31 in the hollow fiber membrane layer are fixed to each other at one end portions and the other end portions thereof. Conditions 30 ‰¤ µ ‰¤ 60 and OD ‰¤ 4.5 × µ are met where µ [µm] is an average separation distance between the adjacent hollow fiber membranes 31 at a fixed portion of the hollow fiber membrane layer and OD [µm] is the outer diameter of the hollow fiber membranes 31,.