The separator for a battery according to the present invention is a separator for a battery including an insulator layer containing a fibrous material having a heat resistant temperature of equal to or higher than 150° C., insulating inorganic fine particles and a binder, or a separator for a battery including a porous layer formed of a thermal melting resin and an insulator layer containing insulating inorganic fine particles and a binder, wherein water content per unit volume is equal to or smaller than 1 mg/cm3 when the separator is held for 24 hours in an atmosphere with a relative humidity of 60% at 20° C. The use of the separator for a battery according to the present invention makes it possible to provide a lithium secondary battery that has favorable reliability and safety and is excellent in storage characteristics and charge-discharge cycle characteristics.