A biological component collection system, comprising: a separation device having a separation processing unit, and adapted to separate a biological component from a liquid containing at least one biological component; and a biological component collection device configured to be attachable to the separation device and having a flow path formed in the interior thereof; wherein the biological component collection device includes a collection line adapted to introduce the liquid containing at least one biological component from a donor, a returning line adapted to return an uncollected biological component to the donor, and a filter structural member provided in the returning line, the collection line and the returning line communicating with each other via a branching member; the separation device includes a collection and returning pump disposed in a line between the separation processing unit and the biological component collection device, and a clamp (72a - 72d) capable of opening and closing an inlet flow path and an outlet flow path of the filter structural member, and configured to, by driving the collection and returning pump, alternately carry out a collection operation of transferring the liquid containing at least one biological component from the donor to the separation processing unit via the collection line, and a returning operation for transferring an unnecessary biological component from the separation processing unit to the donor via the returning line; and the separation device is configured to perform: a pressure release step of stopping a collection and returning pump at an end of the returning operation, and releasing a positive pressure inside the filter structural member; a clamp closure step of, after the pressure release step, closing an inlet flow passage and an outlet flow passage of the filter structural member by the clamp (72a - 72d); and a collection operation starting step of, after the clamp closure step, starting a subsequent collection operation by driving the collection and returning pump. a separation device having a separation processing unit, and adapted to separate a biological component from a liquid containing at least one biological component; and a biological component collection device configured to be attachable to the separation device and having a flow path formed in the interior thereof; wherein the biological component collection device includes a collection line adapted to introduce the liquid containing at least one biological component from a donor, a returning line adapted to return an uncollected biological component to the donor, and a filter structural member provided in the returning line, the collection line and the returning line communicating with each other via a branching member; the separation device includes a collection and returning pump disposed in a line between the separation processing unit and the biological component collection device, and a clamp (72a - 72d) capable of opening and closing an inlet flow path and an outlet flow path of the filter structural member, and configured to, by driving the collection and returning pump, alternately carry out a collection operation of transferring the liquid containing at least one biological component from the donor to the separation processing unit via the collection line, and a returning operation for transferring an unnecessary biological component from the separation processing unit to the donor via the returning line; and the separation device is configured to perform: a pressure release step of stopping a collection and returning pump at an end of the returning operation, and releasing a positive pressure inside the filter structural member; a clamp closure step of, after the pressure release step, closing an inlet flow passage and an outlet flow passage of the filter structural member by the clamp (72a - 72d); and a collection operation starting step of, after the clamp closure step, starting a subsequent collection operation by driving the collection and returning pump. a pressure release step of stopping a collection and returning pump at an end of the returning operation, and releasing a positive pressure inside the filter structural member; a clamp closure step of, after the pressure release step, closing an inlet flow passage and an outlet flow passage of the filter structural member by the clamp (72a - 72d); and a collection operation starting step of, after the clamp closure step, starting a subsequent collection operation by driving the collection and returning pump.