A lithium ion secondary battery, comprising an electrode assembly resulting from stacking a positive electrode, a separator and a negative electrode, wherein the battery comprises, as the separator, a first separator and a second separator having mutually different characteristics, the first separator and the second separator being disposed inside the electrode assembly in a state where the first separator and the second separator are not in contact with each other in a stacking direction of the electrode assembly, the first separator is formed from a polyolefin resin characterised in that the second separator is a nonwoven fabric formed from fibers selected from glass, cellulose or polyacrylonitrile, the first separator and the second separator have the following characteristic: in a case where the battery is constructed comprising an electrode assembly that results from stacking the positive electrode, the first separator and the negative electrode, a resistance increase rate X=Delta X/Delta P satisfies X>0 (positive value), X being evaluated from a change amount Delta P (MPa) of surface pressure applied in the stacking direction of the electrode assembly and a IV resistance increment Delta X (%) of the battery upon application of the surface pressure change amount Delta P, and in a case where the battery is constructed comprising an electrode assembly that results from stacking the positive electrode, the second separator and the negative electrode, a resistance increase rate Y=Delta Y/Delta P satisfies Y<0 (negative value), Y being evaluated from a change amount Delta P (MPa) of surface pressure applied in the stacking direction of the electrode assembly and a resistance increment Delta Y (%) of the battery upon application of the surface pressure change amount Delta P, wherein the IV resistance is calculated on the basis of the difference between the voltage value (initial voltage value) before charge and discharge at a current value of 30C in an environment at 25 degrees Celsius, and a voltage value at a point in time after 10 seconds had elapsed since the initial voltage value, and the resistance increase rate X is calculated from the slopes in the graph that denotes the correlation between surface pressure and IV resistance. in a case where the battery is constructed comprising an electrode assembly that results from stacking the positive electrode, the first separator and the negative electrode, a resistance increase rate X=Delta X/Delta P satisfies X>0 (positive value), X being evaluated from a change amount Delta P (MPa) of surface pressure applied in the stacking direction of the electrode assembly and a IV resistance increment Delta X (%) of the battery upon application of the surface pressure change amount Delta P, and in a case where the battery is constructed comprising an electrode assembly that results from stacking the positive electrode, the second separator and the negative electrode, a resistance increase rate Y=Delta Y/Delta P satisfies Y<0 (negative value), Y being evaluated from a change amount Delta P (MPa) of surface pressure applied in the stacking direction of the electrode assembly and a resistance increment Delta Y (%) of the battery upon application of the surface pressure change amount Delta P, wherein the IV resistance is calculated on the basis of the difference between the voltage value (initial voltage value) before charge and discharge at a current value of 30C in an environment at 25 degrees Celsius, and a voltage value at a point in time after 10 seconds had elapsed since the initial voltage value, and the resistance increase rate X is calculated from the slopes in the graph that denotes the correlation between surface pressure and IV resistance. <EOS>