Washed platelets having a sufficiently low plasma content are more reliably and efficiently obtained. A tertiary separator has a third chamber and includes a main body part 58 formed as a housing part 54a for housing a centrifuged platelet 104, a platelet-containing component 100, and a platelet-added solution 102. An inflow port 77c for inflow, and an outflow port 78a for outflowing plasma, the platelet-added solution 102, and the platelet 104 are provided. Of the wall portions constituting the main body portion 58, at least the bottom portion (first bottom portion 60) forming the accommodating portion 54a is formed of a soft material. [Selection] Figure 5【課題】血漿含有率が充分に低い洗浄血小板をより確実且つ効率的に得る。【解決手段】３次分離器４２は、第３チャンバ５２を有し、遠心分離された血小板１０４を収容する収容部５４ａとして形成された本体部５８と、血小板含有成分１００及び血小板添加溶液１０２を流入させる流入口７７ｃと、血漿、血小板添加溶液１０２及び血小板１０４を流出させる流出口７８ａとを備える。本体部５８を構成する壁部のうち、少なくとも収容部５４ａを形成する部分の底部（第１底部６０）は、軟質素材により形成されている。【選択図】図５
