NOVELTY - An epoxy resin composition containing epoxy resin, curing agent and porogen is prepared. The cured product of the epoxy resin is cut into sheets or the sheet materials of the epoxy resin composition are cured, to obtain epoxy resin sheet (16). The sheet (16) containing porogen is removed using a halogen-free solvent, to obtain epoxy resin porous membrane (17). Infrared rays are irradiated to the obtained porous membrane and absorption of the infrared rays in the membrane is measured, to obtain separator.    USE - Manufacture of separator for nonaqueous electrolyte electrical storage device e.g. lithium ion secondary battery and high capacitance secondary battery. Uses include but are not limited to vehicles, motorcycles, boats and ships, construction machines, industrial machines and electrical storage systems for residences.    ADVANTAGE - The method provides separator with easily controllable parameters, such as average hole diameter and film thickness without using large amounts of solvent.    DETAILED DESCRIPTION - An epoxy resin composition containing epoxy resin, curing agent and porogen is prepared. The cured product of the epoxy resin is cut into sheets or the sheet materials of the epoxy resin composition are cured, to obtain epoxy resin sheet. The epoxy resin sheet containing porogen is removed using a halogen-free solvent, to obtain epoxy resin porous membrane. Infrared rays are irradiated to the obtained epoxy resin porous membrane and absorption of the infrared rays in the epoxy resin porous membrane is measured, to obtain separator. The film thickness and/or average hole diameter of the epoxy resin porous membrane is computed based on infrared rays absorption. An INDEPENDENT CLAIM is included for manufacture of epoxy resin porous membrane.    DESCRIPTION OF DRAWING(S) - The drawing shows the system for the manufacture of the separator.    Epoxy resin sheet (16)    Epoxy resin porous membrane (17)    Mixing apparatus (31)    Hardening material (32)    Cutting apparatus (33)