PROBLEM TO BE SOLVED: To simultaneously achieve both prevention of generation of an aggregate such as a thrombus and air venting.SOLUTION: An air vent chamber includes: a nearly cylindrical chamber body 13 an access part 16 for venting air an inflow port 11 for introducing a liquid such as blood into the chamber body 13, connected to the chamber body 13 at an angle by which the inflow port 11 faces at least a part of the inner wall 14 of the chamber body 13 an outflow port 12 for discharging the liquid such as blood from the chamber body 13 and a protuberance 15 formed in a facing part facing the inflow port 11, of the inner wall 14 of the chamber body 13, and rising toward the inflow port 11. The protuberance 15 is positioned in front of or below the inflow port 11 in the direction where they face each other, and a jet of the blood spouting from the inflow port 11 bumps against the protuberance 15.
