Abstract: A check valve comprising two valve films 11, 12 of synthetic resin that put one on the other, having an air passage 2 defined by two seal sections 21, 22 formed by means of bonding parts of the valve films together and a compression bag and air compression bag equipped with this check valve, wherein the inlet 2a of the air passage 2 is defined by the straight line linking the respective upward flow side end sections 21a, 221a of the seal sections 21, 22; the outlet 2b of the air passage is defined by the straight line linking the respective downward flow side end sections 21b, 223b of the seal sections 21, 22; and in at least one of the seal sections 21, 22, the valve films 11, 12 are not bonded together at the line of extension 21c, 223c of the seal section 21, 22, situated on the downward flow side of the downward flow side end section 21b, 223b of the seal section 21, 22.