To reduce the number of parts to reduce assembling cost by integrating a seal member isolating two oil passages for full-flow and bypass flow with a check valve.

A composite valve disc 30 arranged in the casing of an oil filter is constituted by integrally molding a seal part 34 made of rubber in the vicinity of the leading end of the cylindrical part 33 extended from the base part 32 of a check valve part 31 so as to turn the same toward the inner periphery of the cylindrical part 33, and the position of the seal part 34 is uniquely determined by filing the check valve part 31 to the base part 32. By this constitution, the assembling work of the composite valve disc 30 is performed extremely easily only by inserting and fitting the cylindrical part 33 in the cylindrical end surface of the filter element housed in the oil filter and, therefore, assembling cost is reduced. Further, since the position of the seal part 34 is not moved by an oil pressure change, operation stability is ensured.