Provided is a coin-shaped lithium ion secondary battery including a positive electrode layer, a negative electrode layer, a separator interposed between, an electrolytic solution, and an exterior body having a coin shape with a bulge on at least one surface and comprising a closed space accommodating the positive electrode layer, the negative electrode layer, the separator, and the electrolytic solution. The lithium ion secondary battery has a main region where all of the positive electrode layer, the negative electrode layer, and the separator overlap and a peripheral region which is devoid of at least one of the positive electrode layer, the negative electrode layer, and the separator, wherein a battery bulge ratio that is the ratio of the maximum thickness of the lithium ion secondary battery in the main region to the minimum thickness of the lithium ion secondary battery in the main region is 1.01 to 1.25.