PROBLEM TO BE SOLVED: To provide an air trap chamber capable of reliably preventing blood coagulation by suppressing the diffusion of replenishing fluid in a replenishing fluid layer and the retention of blood in a blood layer.SOLUTION: The air trap chamber comprises: a lower chamber part 5a which includes a first receiving space S1 connected to a blood circuit and being capable of receiving blood circulating through the blood circuit, and for which are formed a blood introduction inlet 5aa capable of introducing blood into the first receiving space S1 and a blood discharge outlet 5ab capable of discharging blood； an upper chamber part 5b which is formed in an upper section of the lower chamber part 5a and includes a second receiving space S2 capable of receiving replenishing fluid, and for which is formed a replenishing fluid introduction inlet 5ba capable of introducing replenishing fluid into the second receiving space S2； and a step part 5c which is located at a boundary between the upper chamber part 5b and the lower chamber part 5a, and formed stepwise such that the upper chamber part 5b side is cross-sectionally smaller than the lower chamber part 5a side.SELECTED DRAWING: Figure 5【課題】補液層の補液の拡散及び血液層の血液の滞留を抑制して血液凝固を確実に防止することができるエアトラップチャンバを提供する。【解決手段】血液回路に接続されて当該血液回路を流通する血液を収容し得る第１収容空間Ｓ１を有するとともに、第１収容空間Ｓ１に対して血液を導入し得る血液導入口５ａａ及び血液を導出し得る血液導出口５ａｂがそれぞれ形成された下部チャンバ部５ａと、下部チャンバ部５ａの上部に形成され、補液を収容し得る第２収容空間Ｓ２を有するとともに、当該第２収容空間Ｓ２に対して補液を導入し得る補液導入口５ｂａが形成された上部チャンバ部５ｂと、上部チャンバ部５ｂと下部チャンバ部５ａとの境界に位置するとともに、当該上部チャンバ部５ｂ側が下部チャンバ部５ａ側よりも断面積が小さくなるように段状に形成された段部５ｃとを有したものである。【選択図】図５
