A
porous membrane of vinylidene
fluoride resin, comprising a substantially single layer membrane of vinylidene
fluoride resin having two major surfaces sandwiching a certain thickness, including a dense layer that has a small pore size and governs a
filtration performance on one major surface side thereof, having an asymmetrical
gradient network structure wherein pore sizes continuously increase from the one major surface side to the other opposite major surface side, and satisfying conditions: (a) the dense layer includes a 5 μm-thick portion contiguous to the one major surface showing a
porosity A1 of at least 60%, (b) the one major surface shows a pore size P1 of at most 0.30 μm, and (c) the
porous membrane shows a ratio Q / P14 of at least 5×104 (m / day·μm4), wherein the ratio Q / P14 denotes a ratio between Q (m / day) which is a value normalized to a whole layer
porosity A2=80% of a water
permeation rate measured at a test length L=200 mm under the conditions of a
pressure difference of 100 kPa and a
water temperature of 25° C., and a fourth power P14 of the pore size P1 on the one major surface. The
porous membrane is produced through a process including: extruding a melt-kneaded mixture of a vinylidene
fluoride resin and a
plasticizer through a die into a form of a film, followed by cooling, to form a solidified film; and extracting the
plasticizer to recover a porous membrane; wherein the
plasticizer is mutually soluble with the vinylidene fluoride resin at a temperature forming the melt-kneaded mixture and further satisfies properties: (i) giving the melt-kneaded mixture with the vinylidene fluoride resin with a
crystallization temperature Tc′ (° C.) which is lower by at least 6° C. than a
crystallization temperature Tc of the vinylidene fluoride alone, (ii) giving the cooled and solidified product of the melt-kneaded mixture a
crystal melting
enthalpy ΔH′ (J / g) of at least 53 J / g per weight of the vinylidene fluoride resin as measured by a differential scanning
calorimeter (DSC), and (iii) the plasticizer alone showing a
viscosity of 200 mPa-s-1000 Pa-s at a temperature of 25° C. as measured according to JIS K7117-2 (using a cone-plate-type
rotational viscometer).