A washed platelet having a sufficiently low blood plasma content rate is more securely and efficiently obtained.A tertiary separator (42) includes a main body (58) which has a third chamber (52) and is formed as an accommodating portion (54a) accommodating a centrifuged platelet (104), an inlet (77c) which allows a platelet containing component (100) and a platelet added solution (102) to flow in, and an outlet (78a) which allows blood plasma, the platelet added solution (102), and the platelet (104) to flow out. A bottom portion (first bottom portion (60)) of at least a portion forming the accommodating portion (54a) in a wall portion included in the main body (58) is formed of a soft material.
