An oxygenator includes: a housing; a bubble-removing hollow fiber membrane layer removing a bubble; a gas-exchanging hollow fiber membrane layer exchanging a gas with blood; and a discharge port to discharge the bubble removed by the bubble-removing hollow fiber membrane layer to the outside of the housing. The oxygenator further includes a gas permeable portion that is arranged between the discharge port and an end portion of the bubble-removing hollow fiber membrane layer, is formed by a member having gas permeability, and allows passage of the bubble removed by the bubble-removing hollow fiber membrane layer without allowing passage of plasma leaking through the bubble-removing hollow fiber membrane layer. A plasma capture chamber that captures the plasma leaking through the bubble-removing hollow fiber membrane layer is formed between the end portion of the bubble-removing hollow fiber membrane layer and the gas permeable portion.