Provided are: a fouling inhibitor that is capable of effectively inhibiting fouling of porous filtration membranes use as water purification membranes or the like, that has resistance to chemicals, such as alkali used when the membrane is fouled, and that is capable of maintaining such effects sufficiently even after chemical wash; a filtration membrane provided with the inhibitor; and a method for producing the same. The inhibitor contains:
(A) a copolymer having a weight average molecular weight of 10000 to 300000 and contains a MPC unit (a1) and a BMA unit (a2), wherein the ratio of (a1) to (a2) ((a1)/(a2)) bymole is 10/90 to 70/30; and (B) PVAhaving a saponification degree of 72 to 93 mol% and a polymerization degree of 300 to 1000, wherein the ratio of component (A) to component
(B) ((A)/(B)) by mass is 25/75 to 75/25.