NOVELTY - A multilayer microporous membrane has several thin layers (T1) containing a polymer (A) and having thickness of 1 mu m or less, and several thin layers (T2) containing a polymer (B) and having thickness of 1 mu m or less. The polymer (A) contains polymethyl pentene (20 weight% or more). The polymer (B) contains a polymer (more than 80 weight%) incompatible to polymer (A). The multilayer microporous membrane is liquid permeable membrane.    USE - Multilayer microporous membrane is used for battery separator film for battery e.g. lithium ion secondary battery, lithium polymer secondary battery, nickel-hydrogen secondary battery, nickel cadmium secondary battery, nickel zinc secondary battery, or silver zinc secondary battery (all claimed). Uses include but are not limited to power supply for electric vehicles and hybrid electric vehicles.    ADVANTAGE - The multilayer microporous membrane has excellent physical properties including permeability is manufactured without causing delamination in the film.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) manufacture of multilayer microporous membrane, which involves forming primary layered material containing primary layer and secondary layer formed using solutions obtained by mixing polymers (A,B) and diluent (D1,D2), respectively, forming secondary layered material by decreasing thickness of primary layered material and increasing number of layers, expanding width of primary layered material and/or decreasing thickness of secondary layered material, expanding width of secondary layered material, and removing portion of diluent; and    (2) battery, which has separator provided between positive electrode and negative electrode, and an electrolyte. The separator has the membrane.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic view of extrusion system used for manufacturing multilayer microporous membrane.    Primary mixture (100)    Extruders (101,103)    Secondary mixture (102)    Feed block (104)    Multi-layer-extrusion product (105)