Provided is a blood component collection cassette, a blood component collection kit, a blood component collection system, and a flow path internal pressure detection method capable of accurately measuring a circuit internal pressure in negative pressure and positive pressure regions.A flow path (42) formed in a cassette body (40) of a blood component collection cassette (28) has a first line (42a) through which blood flows when a blood component separation device is in operation, and a second line (42b) through which blood does not flow when the blood component separation device is in operation. A first line forming member (54) is provided with a first pressure-receiving portion (60) pressed by a first load detector (88) attached to the blood component separation device. A second line forming member (56) is provided with a second pressure-receiving portion (62) pressed by a second load detector (90) attached to the blood component separation device.
