Provided are a plasma or serum separator and a plasma or serum sampling method capable of isolating plasma or serum with good efficiency from a small amount of blood without using a centrifuge and without causing leakage of a blood cell component or hemolysis, and in addition, capable of isolating and collecting plasma or serum from a whole blood test sample in a short time with simplicity in a blood test in the scene of medical care requiring an instant treatment any time such as an emergency test, home-use test or the like. The plasma or serum separator for isolating plasma or serum from whole blood comprises: a blood separation member, a holding member covering and holding the blood separation member; a blood introducing portion formed in a portion of the holding member covering a proximal end portion of the blood separation member; and a plasma or serum sampling aperture formed in a portion of the holding member covering a distal end portion of the blood separation member, wherein the whole blood is introduced into the blood separation member through the blood introducing portion, the introduced whole blood is separated such that the plasma or serum is located in the distal end portion of the blood separation member, while blood cells are located in the proximal end portion of the blood separation member; thereby enabling the plasma or serum located in the distal end portion of the blood separation member to be sampled through the plasma or serum sampling aperture.