A separator, used for a lithium ion secondary battery that produces electromotive force by lithium doping/dedoping, which is composed of a porous film comprising an organic polymer (component A) which can swell in an electrolyte solution to retain it, surrounding a nonwoven fabric which cannot swell in the electrolyte solution, the porous film including an electrolyte solution non-swelling organic polymer with a melting point of 210.degree. C. or above (component B) and an electrolyte solution non-swelling organic polymer with a melting point of 180.degree. C. or below (component C), wherein component B is incorporated in the fiber composing the nonwoven fabric. The separator is highly safe and exhibits low reduction mechanical properties with respect to organic solvents.