To provide a check valve capable of sealing and opening certainly even if fluid pressure is low.

The check valve comprises a female exterior body 12 to form the inlet passage 13, a male exterior body 19 to form the outlet passage 21 and a diaphragm 26 arranged between exterior bodies 12, 19, permitting passage of the fluid from the inlet passage 13 to the outlet passage 21 and regulating passage of the fluid from the outlet passage 21 to the inlet passage 13. The diaphragm 26 includes a communication part 29 having the communication hole 28 allowing communication of the annular fixing part 27, the inlet passage 13 and the outlet passage 21 to allow passage of the fluid and a valve part 30 capable of elastically deforming in the shape of a thin film to cover between the communication part 29 and the holding parts 27. A sealing part 16, which gets into contact (close contact) with the communication part 29 to close the communication hole 28 when the fluid pressure is applied from the outlet passage 21 side, is provided in the inlet passage 13.