A blood bag system (10) has a clamp (64) installed in a flexible first tube (56). The clamp (64) includes: a base section (140) having a protrusion section (156), an opening/closing section (144) having a pressing protrusion section (170) that presses a side surface of the first tube (56), and a bent section (146) that connects the base section (140) and opening/closing section (144). The base section (140) is provided with a pair of cutout sections (158, 160) opened to a side surface (141) where the first tube (56) can be inserted by interposing the protrusion section (156). The clamp (64) is configured such that a portion of the first tube (56) where the clamp (64) is installed is bent while the clamp (64) is released.
