TECHNOLOGY FOCUS - POLYMERS - Preferred Composition: The mixture (A) contains 1-40% of components having molecular weight of 1000000 or less, and 1-40% of components having molecular weight of 10000 or less, as measured by gel permeation chromatography. The high-density polyethylene having Mv of 500000-5000000 in the film, is polyethylene having Mv of 1500000 or more. The polyethylene contained in the microporous polyethylene film, is mixture of polyethylene (A) having Mv of 150-5000000, polyethylene (B) having Mv of 60-1500000 and polyethylene (C) having Mv of 25-600000. Preferred olefin: The (alpha)-olefin contained in the microporous polyethylene film is propylene. Preferred Properties: The micro porous polyethylene film has shrinkage force of 2 N or less at 150degreesC, fuse temperature of 140degreesC or less, temperature of 150degreesC or more, thickness of 5-24 mum, porosity of 30-70% and air permeability of 100-600 seconds. EXAMPLE - High-density ethylene copolymer (in mass parts) (10.5) having melt index of 0.8, density of 0.95, propylene unit content of 0.6 mol% and Mv of 150000, high-density homo polyethylene (10.5) having melt index of 0.05, density of 0.95, and Mv of 300000, high-density polyethylene (5.2) having melt index of 0.01, density of 0.95 and Mv of 700000, ultrahigh molecular weight homo polyethylene (8.8) having density of 0.94 and Mv of 2000000, and tetrakis-(methylene-3-(3', 5'-di-t-butyl-4'-hydroxy phenyl)propionate) methane (0.3), were mixed. The obtained mixture was supplied to twin-screw extruder via feeder. Then, liquid paraffin (65) was supplied to the extruder by side feed, and extruded from T-die provided in the tip of extruder. The extruded material was solidified by cooling, to form gel-like sheet of 1200 mum thickness. The obtained gel-like sheet was drawn by biaxial stretching machine at 120degreesC. The obtained oriented film was immersed in methyl ethyl ketone, and dried after removing liquid paraffin, to obtain microporous polyethylene film. The obtained film was heat-set at 125degreesC. The film had 7% of components having molecular weight of 1000000 or more and 5% of component having molecular weight of 10000 or less. The obtained film had Mv of 500000, comonomer content (3C or more (alpha)-olefin content) of 0.2 mol%, thickness of 17 mum, porosity of 40%, air permeability of 400 seconds, puncture strength of 5.1 N, fuse temperature of 137degreesC, film breaking temperature 157degreesC, shrinkage force of 0.5 N at 150degreesC, and contraction stress of 99 kPa at 150degreesC. A battery was formed using the obtained microporous polyethylene film as separator. When overcharge test of battery was carried out, the generation of heat was not observed.