The present invention relates to a lithium ion secondary battery comprising an electrode element comprising a positive electrode, a negative electrode and a separator, and an electrolyte solution, wherein the separator has a shrinking ratio of 2% or less by heat treatment at 90 ° C. for 6 hours, and a contest of physically adsorbed water of the electrode element is 2% by mass or less, or a content of chemically adsorbed water in a positive electrode active material layer of the positive electrode is 1% by mass or less.