A biological component sampling cassette including a cassette main body provided with a plurality of flow paths and a plurality of port members, the flow paths connecting the port members, and configured to be attachable to a separation device configured to separate a biological component from liquid containing at least one biological component, wherein the cassette main body is made of a soft material to which heat sterilization is applicable, the plurality of flow paths includes an introduction line configured to introduce the liquid, a biological component transfer line configured to transfer, to a sampling container (30, 32, 34), the biological component obtained by separation processing of the liquid, and a return line configured to transfer liquid other than the biological component obtained by the separation processing, the return line is provided with a reservoir configured to temporarily store the liquid to be returned, and the reservoir is configured to be expandable and contractable, and is configured to be pressed by a return pump provided at the separation device to discharge the liquid from the reservoir, characterized in that the cassette main body is made of two sheets (38, 39) of a soft material, the cassette main body includes a sensor pressing section that is configured to press a reservoir pressure sensor equipped at the separation device, to thereby detect an inner pressure of the reservoir, wherein the sensor pressing section is located in a portion of the return line that is connected between the reservoir and two port members of the cassette main body, the two port members being configured to be connected with external tubes (20f, 20g), the two port members are at an edge of the cassette main body, the sensor pressing section is provided at a flow path joint between two flow path elements (44a, 44b) respectively connected to the two ports, and the sensor pressing section is configured to be expandable and contractable according to an inner pressure. wherein the cassette main body is made of a soft material to which heat sterilization is applicable, the plurality of flow paths includes an introduction line configured to introduce the liquid, a biological component transfer line configured to transfer, to a sampling container (30, 32, 34), the biological component obtained by separation processing of the liquid, and a return line configured to transfer liquid other than the biological component obtained by the separation processing, an introduction line configured to introduce the liquid, a biological component transfer line configured to transfer, to a sampling container (30, 32, 34), the biological component obtained by separation processing of the liquid, and a return line configured to transfer liquid other than the biological component obtained by the separation processing, the return line is provided with a reservoir configured to temporarily store the liquid to be returned, and the reservoir is configured to be expandable and contractable, and is configured to be pressed by a return pump provided at the separation device to discharge the liquid from the reservoir, characterized in that the cassette main body is made of two sheets (38, 39) of a soft material, the cassette main body includes a sensor pressing section that is configured to press a reservoir pressure sensor equipped at the separation device, to thereby detect an inner pressure of the reservoir, wherein the sensor pressing section is located in a portion of the return line that is connected between the reservoir and two port members of the cassette main body, the two port members being configured to be connected with external tubes (20f, 20g), the two port members are at an edge of the cassette main body, the sensor pressing section is provided at a flow path joint between two flow path elements (44a, 44b) respectively connected to the two ports, and the sensor pressing section is configured to be expandable and contractable according to an inner pressure.