Abstract: In a lithium ion secondary battery, a lithium-nickel-containing composite oxide that contains lithium and nickel is used as a positive electrode active material, and a negative electrode active material having a melting temperature of 1200° C. or less in a lithium-absorbed state is included in a negative electrode active material layer. By configuring as above, in the lithium ion secondary battery, it is possible to achieve higher capacity, higher output, and longer life, as well as further improved safety, and particularly in a nail penetration test, to suppress the heat generation due to internal short circuit and to reliably prevent the occurrence of thermal runaway.