Problem: The invention provides a separator for a lithium-ion secondary battery exhibiting excellent strength, wettability with nonaqueous electrolyte solutions, voltage endurance and cycle characteristics in lithium-ion secondary batteries, and a method of increasing the puncture depth of the separator. Solution: A separator for a lithium-ion secondary battery is formed of a microporous film comprising a polyolefin resin (A) as a major component, and a resin (B), at least portions of the surfaces of the micropores in the microporous film being coated with resin (B).