Nonwoven fabric and battery separator

A battery separator consisting of a single layer of a non-woven fabric having a thickness of 60 μm or less, the non-woven fabric being formed by integrating a composite fiber of a core-sheath structure consisting of a core part and a sheath part covering the core part by fusion-bonding the sheath parts each other, wherein

• the composite fiber has a fiber diameter of 3.4 to 4.5 μm, the core part is formed of a first polyolefin-based resin, the sheath part is formed of a second polyolefin-based resin having a melting point lower than that of the first polyolefin-based resin,
• a basis weight is 3 to 20 g/m2, and
• an amount of strain, S, as calculated by a following formula (I)
• S (mm2)=(MD1−MD0)×(CD0−CD1) (I)
• is 50 mm2 or less wherein MD1 (mm) and CD1 (mm) are lengths in MD direction and CD direction respectively when a tensile stress of 20 N in MD direction is applied to a sample having an effective length in MD direction (MD0) of 100 mm and an effective length in CD direction (CD0) of 100 mm under an environment of a room temperature of 25° C. and a humidity of 50% RH.