This blood component separating device (1) is provided with: a blood retaining tank (20) having a first retaining part (21) and a second retaining part (22)； a slider (30) that is movable from the first retaining part to the second retaining part； and a flow path (40f) that communicates the interior of the retaining tank with the outside. When the slider is located in the first retaining tank, the first retaining tank and the second retaining tank are in communication with each other. When the slider is inserted into the second retaining part, a liquid-tight seal is formed between the slider and the inner circumferential surface of the second retaining part and the slider inhibits communication between the first retaining part and the second retaining part. The slider is capable of moving in the second retaining tank while maintaining the liquid-tight seal between the slider and the inner circumferential surface of the second retaining part. As the slider penetrates into the second retaining part, blood components in the second retaining part are pressed out of the retaining tank through the flow path.血液成分分離用裝置(1)，具備具有第1貯存部(21)及第2貯存部(22)之血液貯存槽(20)、能夠自第1貯存部往第2貯存部移動之滑塊(30)、及使貯存槽之內外連通之流路(40f)。於滑塊處於第1貯存部內時，第1貯存部與第2貯存部連通。當將滑塊插入至第2貯存部時，於滑塊與第2貯存部之內周面之間形成液密之密封，且藉由滑塊將第1貯存部與第2貯存部之連通阻斷。滑塊可一邊於與第2貯存部之內周面之間維持液密之密封，一邊於第2貯存部內移動。隨著滑塊進入至第2貯存部內，第2貯存部內之血液成分通過流路而被擠出至貯存槽外。
