Provided is a drug solution circuit capable of more reliably preventing backflow of blood that can be installed easily on an injection device. A drug solution circuit 500 is equipped with a first base line 508 through which a first drug solution flows, a second base line 509 through which a second drug solution flows, a subject line 503 connected to the first base line 508 and the second base line 509, and a first closing part 100a having a first closing mechanism 120a for closing an internal circuit. The first closing mechanism 120a is equipped with a first moving member 122a1 and a second moving member 122a2, each having a flow path, and a first housing 124a that slidably accommodates the first moving member 122a1 and the second moving member 122a2. The circuit is configured so that the flow path of the second moving member 122a2 is opened after the flow path of the first moving member 122a1 has opened.提供一種能夠更為確實地防止血液的倒流且能夠更容易裝設於注入裝置的藥液迴路。藥液迴路(500)，係構成為具備：第1基線(508)，係流動有第1藥液；第2基線(509)，係流動有第2藥液；受試者線(503)，係連接至第1基線(508)及第2基線(509)；以及第1閉鎖部(100a)，係具有將內部流路閉鎖的第1閉鎖機構(120a)；第1閉鎖機構(120a)，係具備分別具有流路的第1移動構件(122a1)及第2移動構件(122a2)，以及將第1移動構件(122a1)及第2移動構件(122a2)以能夠滑動的方式收容的第1殼體(124a)，在將第1移動構件(122a1)的流路開放之後，將第2移動構件(122a2)的流路開放。
