[Problem] To provide an artificial lung capable of increasing the speed of priming and restricting leakage of plasma. [Solution] An artificial lung 10 has: a housing 20; an air bubble removing hollow fiber membrane layer 31 for removing air bubbles; a gas exchange hollow fiber membrane layer 41 for exchanging gas in blood; and a discharge port 50 for discharging air bubbles removed by the air bubble removing hollow fiber membrane layer to the outside of the housing. The artificial lung further has a gas transmission part 60 which is disposed between the discharge port and an end of the air bubble removing hollow fiber membrane layer, is formed of a gas permeable member, does not allow plasma leaking through the air bubble removing hollow fiber membrane layer to pass therethrough, and allows air bubbles removed by the air bubble removing hollow fiber membrane layer to pass therethrough. A plasma collecting chamber 70 is defined and formed between the end of the air bubble removing hollow fiber membrane layer and the gas transmission part, and collects plasma leaking through the air bubble removing hollow fiber membrane layer.