NOVELTY - The separator (30) is provided with an upper porous layer (31) and a lower porous layer (32). The upper porous layer is provided with polypropylenes. The lower porous layer is provided with polyethylene or a polypropylene. An interlayer (34,35) is formed between the upper porous layer and lower porous layer. The interlayer is equipped with a complex oxide particle (36) containing calcium (Ca), sodium (Na), and aluminum (Al) and silicon (Si). A heat-resistant porous layer (38) is laminated on the surface of the upper porous layer or lower porous layer.    USE - Separator arranged between positive and negative electrodes of cylindrical shaped lithium ion secondary battery (claimed) used as driving power supply of portable electronic devices, such as hybrid vehicle, notebook computer, and video camcorder.    ADVANTAGE - The shutdown characteristic of a separator can be made favorable, by arranging the complex oxide particle to the interlayer of upper porous layer and a lower porous layer. The malfunction produced in location where thickness becomes thin extremely in a lower porous layer can be prevented, and the shutdown resistance of a separator can be raised. The separator is provided with the increased ability to prevent the movement of ion, so that shutdown resistance can be made strong. The short circuit expansion and fine short circuit generated between electrodes can be prevented even when a foreign material penetrate a separator, by providing a heat-resistant porous layer in the surface of a upper porous layer or lower porous layer. The reliability of the shutdown function of a separator can be improved, so that the thermal runaway of a battery can be prevented. Thus the lithium ion secondary battery is provided with high safety.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for lithium ion secondary battery.    DESCRIPTION OF DRAWING(S) - The drawing shows the expanded sectional view of the separator.    Separator (30)    Upper porous layer (31)    Lower porous layer (32)    Interlayer (34,35)    Complex oxide particle (36)    Heat-resistant porous layer (38)