A filter member (4) used in an oxygenator is constructed to provide improved contact with a hollow fiber membrane bundle (3) and to capture bubbles contained in blood. The filter member (4) possesses elasticity at least in the circumferential direction to allow the inner circumference of the filter member to be increased from a natural non-expanded state prior to placement on the hollow fiber membrane bundle to an expanded state in which the inner circumference of the filter member is increased when placed on the hollow fiber membrane bundle. The filter member is constructed to satisfy the condition 0.5 ‰ L2/L1 < 1, wherein L1 represents the outer circumference of the hollow fiber membrane bundle and L2 represents the inner circumference of the filter member in the natural non-expanded state.
