NOVELTY - A polyolefin composition (a1) containing polyethylene and polypropylene and diluent (a2) are mixed, to obtain reaction mixture (A). Then, polyolefin composition (b1) containing polyethylene and diluent (b2) are mixed, to obtain reaction mixture (B). The obtained reaction mixture (A) is extruded, cooled and orientation is carried out, to obtain reaction material (C). The obtained reaction mixture (B) is extruded, cooled and orientation is carried out, to obtain reaction material (D). Thus, manufacture of multilayer microporous membrane (18) is carried out.    USE - Manufacture of multilayer microporous membrane used as separator film for battery such as lithium ion secondary battery used as power source for electric vehicle and hybrid electric vehicle (all claimed). Can also be used for lithium-polymer secondary batteries, nickel-hydrogen secondary batteries, nickel-cadmium secondary batteries, nickel-zinc secondary batteries and silver-zinc secondary batteries.    ADVANTAGE - The method provides multilayer microporous membrane having improved thickness, air permeability, pin puncture strength, tensile strength, tensile elongation, heat shrinkage property and melt down temperature. The battery formed using the multilayer microporous membrane has improved battery recovery ratio and melt down temperature.    DETAILED DESCRIPTION - A polyolefin composition (a1) containing polyethylene having crystal dispersion temperature (Tcd) and polypropylene and diluent (a2) are mixed, to obtain reaction mixture (A). Then, polyolefin composition (b1) containing polyethylene having crystal dispersion temperature and diluent (b2) are mixed, to obtain reaction mixture (B). The obtained reaction mixture (A) is extruded, cooled and orientation is carried out in direction(s) of 1-10 fold at Tcd plus minus 15 degrees C, to obtain reaction material (C). The obtained reaction mixture (B) is extruded, cooled and orientation is carried out in direction(s) of 1-5 fold at temperature more than 10-40 degrees C of Tcd plus minus 15 degrees C, to obtain reaction material (D). Thus, manufacture of multilayer microporous membrane (18) is carried out. The polyolefin composition (b1) further contains polypropylene. The polyethylene is high-density polyethylene. The polypropylene is high-density polypropylene. The polyolefin composition (a1) and polyolefin composition (b1) further contain high-molecular-weight polyethylene. A battery is formed using anode, cathode and separator having the multilayer microporous membrane.    DESCRIPTION OF DRAWING(S) - The drawing shows schematic view of system for producing biaxially-oriented coextruded multilayer film or sheet.    Extruders (2,12)    Feed hoppers (8,15)    Solvent feedlines (6,16)    Multilayer sheet (18)    Co-extrusion die (20)