The present invention pertains to a resin composition which includes component (A): a membrane-forming polymer, component (B): a polymer obtained by polymerizing a monomer composition which includes a (meth)acrylic ester macromonomer (b1) represented by general formula (1) and another monomer (b2), and component (C): a polymer including a vinylpyrrolidone unit, a membrane-forming stock solution which includes the resin composition, a porous membrane obtained by forming with the membrane-forming stock solution, and a hollow fiber membrane, a water treatment device, an electrolyte support, and a separator which use the porous membrane. According to the present invention, it is possible to provide a porous membrane which has pores with high uniformity wherein the formation of large pores with a diameter of 1 [mu]m or higher is suppressed and which has excellent fractionation performance and high water permeability, and a hollow fiber membrane, water treatment device, electrolyte support, and separator which use the porous membrane.