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.
