Disclosed is a battery including a casing including cells housed therein and a terminal part including a connection terminal connected to an electrode terminal of a connection apparatus, in which in the casing, a functional groove having a predetermined function in which a longitudinal direction coincides with a connection direction of the connection terminal and the electrode terminal is formed, and the functional groove includes a plurality of functional units having different lengths which are continuously formed. Accordingly, since there is no part that divides a functional unit between a plurality of functional units having different lengths, a size of the functional groove in a direction in which the functional units are continuous is a sum of sizes of a plurality of functional units, and thus it is possible to reduce the size of the battery while securing high functionality.