Porous membrane

A porous membrane containing:

• a hydrophobic polymer;
• and
• a water-insoluble hydrophilic polymer, the porous membrane having:
• a dense layer in a downstream portion of filtration in the membrane;
• a gradient asymmetric structure wherein an average pore diameter of pores increases from the downstream portion of filtration toward an upstream portion of filtration in the membrane;
• a gradient index of the average pore diameter from the dense layer to a coarse layer of 0.5 to 12.0;
• a porosity in the dense layer is 35.1% or more and 45.0% or less;
• and
• a value of a standard deviation of pore diameters/the average pore diameter in the dense layer is 0.85 or less.