An oxygenator comprising: a gas exchange unit having hollow fiber membranes (28a, 30a) being configured to allow blood to undergo gas exchange; and a housing provided with a gas inflow port and a gas outflow port, wherein the housing is comprised of a housing main body with the gas exchange unit accommodated therein, a first cover member disposed on one end portion of the housing main body such that the gas exchange unit is covered at an inlet side end face (29a, 31a) thereof where gas inlets of the hollow fiber membranes (28a, 30a) are located, and a second cover member disposed on another end portion of the housing main body such that the gas exchange unit is covered at an outlet side end face (29b, 31b) thereof where gas outlets of the hollow fiber membranes (28a, 30a) are located, wherein the second cover member includes a cover main body facing the outlet side end face (29b, 31b) of the gas exchange unit, and a pressure control hole formed through the cover main body to control a pressure of gas guided out from the gas exchange unit, the oxygenator further comprising a sampling port and characterized in that the second cover member further includes a recessed wall portion which is recessed relative to the cover main body to be closer to a side of the gas exchange unit than an inner surface of the cover main body, and in that the sampling port is disposed in the recessed wall portion to collect the gas guided out from the gas exchange unit, wherein the sampling port faces, at an inner opening portion thereof, the outlet side end face (29b, 31b) of the gas exchange unit. a gas exchange unit having hollow fiber membranes (28a, 30a) being configured to allow blood to undergo gas exchange; and a housing provided with a gas inflow port and a gas outflow port, wherein the housing is comprised of a housing main body with the gas exchange unit accommodated therein, a first cover member disposed on one end portion of the housing main body such that the gas exchange unit is covered at an inlet side end face (29a, 31a) thereof where gas inlets of the hollow fiber membranes (28a, 30a) are located, and a second cover member disposed on another end portion of the housing main body such that the gas exchange unit is covered at an outlet side end face (29b, 31b) thereof where gas outlets of the hollow fiber membranes (28a, 30a) are located, wherein the second cover member includes a cover main body facing the outlet side end face (29b, 31b) of the gas exchange unit, and a pressure control hole formed through the cover main body to control a pressure of gas guided out from the gas exchange unit, the oxygenator further comprising a sampling port and characterized in that the second cover member further includes a recessed wall portion which is recessed relative to the cover main body to be closer to a side of the gas exchange unit than an inner surface of the cover main body, and in that the sampling port is disposed in the recessed wall portion to collect the gas guided out from the gas exchange unit, wherein the sampling port faces, at an inner opening portion thereof, the outlet side end face (29b, 31b) of the gas exchange unit. a housing main body with the gas exchange unit accommodated therein, a first cover member disposed on one end portion of the housing main body such that the gas exchange unit is covered at an inlet side end face (29a, 31a) thereof where gas inlets of the hollow fiber membranes (28a, 30a) are located, and a second cover member disposed on another end portion of the housing main body such that the gas exchange unit is covered at an outlet side end face (29b, 31b) thereof where gas outlets of the hollow fiber membranes (28a, 30a) are located, wherein the second cover member includes a cover main body facing the outlet side end face (29b, 31b) of the gas exchange unit, and a pressure control hole formed through the cover main body to control a pressure of gas guided out from the gas exchange unit, the oxygenator further comprising a sampling port and characterized in that the second cover member further includes a recessed wall portion which is recessed relative to the cover main body to be closer to a side of the gas exchange unit than an inner surface of the cover main body, and in that the sampling port is disposed in the recessed wall portion to collect the gas guided out from the gas exchange unit, wherein the sampling port faces, at an inner opening portion thereof, the outlet side end face (29b, 31b) of the gas exchange unit.