An extracorporeal circuit (1), which makes it possible to efficiently collect priming solution (P) by displacing the priming solution (P) with blood (B) after the priming solution (P) is fed to the extracorporeal circuit (1) to prime the extracorporeal circuit (1), includes a circuit body (9) including first and second tubes, a feed bag (6) that feeds the priming solution (P), a blood reservoir (3,3A,3B) in which a liquid is temporarily stored, a third tube (10) that branches out from the middle of the first tube, and a collection bag (7,7A) which communicates with the third tube (10) and into which the priming solution (P) is collected through the third tube. After the priming solution (P) is fed from the feed bag (6) to prime at least a housing (33), when the priming solution (P) in the housing (33) is displaced by blood (B), the priming solution (P) in the housing (33) is transferred and collected in the collection bag (7,7A) due to a height difference between the liquid level (L1) and the collection bag (7).
