Provided is a porous membrane having a high blocking performance suited for filtration application as well as being excellent in water permeability performance, which is capable of providing stable filtering operation over a long period of time. The porous membrane has a cross-sectional pore size index of 40 or more in a region where a membrane thickness position ranges from 0.1 to 0.2 within the membrane thickness which is normalized by defining one surface position as 0 and the other surface position as 1 and divided into ten regions in the membrane thickness direction, the cross-sectional pore size index being calculated by: Cross-Sectional Pore Size Index=(Cross-Sectional Pore Size in the Range)/(Pore Size of the One Surface).