To provide an air-trap chamber with which air bubbles contained in a predetermined liquid can be removed to a higher extent and more assuredly and whose chamber portion can therefore have a reduced capacity. An air-trap chamber 5 connected to a blood circuit in which blood is made to flow. The air-trap chamber 5 includes a chamber portion 5c capable of storing the blood by a predetermined amount, a blood inlet 5aa from which the blood in the blood circuit is allowed to be introduced into the chamber portion 5c, and a blood outlet 5ca from which the blood in the chamber portion 5c is allowed to be discharged into the blood circuit. The air-trap chamber 5 is configured to collect and remove air bubbles contained in the blood stored in the chamber portion 5c. The air-trap chamber 5 is characterized in comprising a wall part 7 that allows the blood introduced from the blood inlet 5aa to flow upward and then to flow downward into the chamber portion 5c.
