To provide a check valve capable of surely avoiding a break of a diaphragm even in the case of using in an environment, in which a shocking internal pressure fluctuation is applied from a secondary side, and to provide a water softening system using the same.

A valve element 9 is structured of a diaphragm 91 and a diaphragm plate 92. An intrusion preventing part 925 is formed to be projected from the most outside peripheral position of a peripheral flange 924 of the diaphragm plate toward the diaphragm. Even if a folding part 912 of the diaphragm is swelled upward by receiving the secondary side pressure P2, the folding part 912 is made to abut on the intrusion preventing part to prevent swelling thereof more and to prevent outbreaks of intrusion into a gap S.