Disclosed is a separator for nonaqueous electrolyte secondary batteries, which comprises a first porous layer (layer A) with shut down characteristics which becomes a substantially nonporous layer at high temperatures, and a second porous layer (layer B) which contains an aramid resin and an inorganic material. The ratio of the thickness (TA) of the layer A to the thickness (TB) of the layer B, namely (TA/TB), is not less than 2.5 but not more than 13.