A lithium ion secondary battery includes a positive electrode, a negative electrode, and an electrolytic solution. The positive electrode contains a lithium composite oxide. The negative electrode contains a material including at least one of silicon Si and tin Sn as a constituent element. The lithium composite oxide includes lithium Li having a composition ratio a, a first element having a composition ratio b, and a second element having a composition ratio c as a constituent element. The first element including two kinds or more selected from among manganese Mn, nickel Ni, and cobalt Co, and including at least manganese. The second element including at least one kind selected from among aluminum Al, titanium Ti, magnesium Mg, and boron B. The composition ratios a to c satisfy the relationships of 1.1 b+c.