FLAT SECONDARY BATTERY

A flat secondary battery includes an outer package member, a battery device, and an electrode terminal. The outer package member has a flat and columnar shape, and includes a pair of bottom parts and a sidewall part. The bottom parts are opposed to each other. The sidewall part lies between the bottom parts. At least a portion of a surface of the sidewall part is a curved surface. The battery device is contained inside the outer package member and includes a positive electrode and a negative electrode. The electrode terminal is provided to be exposed at the sidewall part and is coupled to one of the positive electrode and the negative electrode.