An oxygenator, comprising: 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, wherein the hollow fiber membranes in the hollow fiber membrane layer are fixed to each other at one end portions and the other end portions thereof, and wherein condition a) 30 ≤ ε ≤ 60 and OD ≤ 4.5 × ε is 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 membranes, the outer shape of the oxygenator is columnar, the oxygenator is a heat exchanger-equipped oxygenator including a heat exchange portion provided inside thereof, and an oxygenating portion provided on an outer peripheral side of the heat exchange portion and configured to conduct gas exchange with blood, the oxygenator has a housing in which the oxygenating portion and the heat exchange portion are stored, the heat exchange portion is further stored in a heat exchanger housing in the housing, and the heat exchange portion has opposite ends fixed to the housing by the heat exchanger housing, wherein condition b) 0.55 × OD ≤ ID ≤ 0.8 × OD is met where ID [μm] is the inner diameter of the hollow fiber membranes, characterized in that condition c) L ≤ 1.3 × ID is met where ID [μm] is the inner diameter of the hollow fiber membranes and L [mm] is the length of the hollow fiber membranes contributing to gas exchange, and wherein, when the conditions 120 ≦OD ≦5220, (a), (b) and (c) are met and blood is introduced into the housing, the housing is configured to be fillable with an the amount of blood of 25 to 50 ml/m 2 per unit membrane area. 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, wherein the hollow fiber membranes in the hollow fiber membrane layer are fixed to each other at one end portions and the other end portions thereof, and wherein condition a) 30 ≤ ε ≤ 60 and OD ≤ 4.5 × ε is 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 membranes, the outer shape of the oxygenator is columnar, the oxygenator is a heat exchanger-equipped oxygenator including a heat exchange portion provided inside thereof, and an oxygenating portion provided on an outer peripheral side of the heat exchange portion and configured to conduct gas exchange with blood, the oxygenator has a housing in which the oxygenating portion and the heat exchange portion are stored, the heat exchange portion is further stored in a heat exchanger housing in the housing, and the heat exchange portion has opposite ends fixed to the housing by the heat exchanger housing, wherein condition b) 0.55 × OD ≤ ID ≤ 0.8 × OD is met where ID [μm] is the inner diameter of the hollow fiber membranes, characterized in that condition c) L ≤ 1.3 × ID is met where ID [μm] is the inner diameter of the hollow fiber membranes and L [mm] is the length of the hollow fiber membranes contributing to gas exchange, and wherein, when the conditions 120 ≦OD ≦5220, (a), (b) and (c) are met and blood is introduced into the housing, the housing is configured to be fillable with an the amount of blood of 25 to 50 ml/m 2 per unit membrane area.