A blood component separator (1) includes a blood storage vessel (20) that includes a first storage part (21) and a second storage part (22), a slider (30) movable from the first storage part to the second storage part, and a flow path (40f) for communicating an inside and an outside of the storage vessel. When the slider is in the first storage part, the first storage part and the second storage part are in communication with each other. When the slider is inserted into the second storage part, a liquid-tight seal is formed between the slider and the inner peripheral surface of the second storage part and the communication between the first storage part and the second storage part is blocked by the slider. The slider is movable in the second storage part while maintaining the liquid-tight seal between the slider and the inner peripheral surface of the second storage part. As the slider enters the second storage part, the blood component in the second storage part is pushed out of the storage vessel through the flow path.
