The invention relates to a porous membrane having excellent durability and being useful as an ultrafiltration membrane, which substantially comprises a phenylene sulfide copolymer represented by the formula (-Ph-S-)m(-Ph-X-)n and which has a nonuniform structure wherein a dense layer or layers substantially governing the separating function of the membrane are formed on one side or both sides of the membrane and a porous layer having a larger void volume is formed in contact with said dense layer. In said formula Ph represents a phenylene group, X represents -SO2 - or -CO-, and m and n each represents a natural number.