A battery (B) includes electrodes (11a, 11b) and engaging portions (10a, 10b) formed by recessing portions of the battery (B) around the electrodes (11a, 11b), and recesses (14a, 14b, 23a, 23b) are formed in side surfaces of the engaging portions (10a, 10b). The recesses (14a, 14b, 23a, 23b) are symmetrically arranged in the left and right engaging portions (10a, 10b). Transversely symmetrical identification pieces (19a, 19b) project from the leading ends of terminals (15a, 15b). In the case of a correct correspondence of the electrodes (11a, 11b) and the terminals (15a, 15b), the identification pieces (19a, 19b) fit into the recesses (14a, 14b, 23a, 23b), enabling assembling of the terminals (15a, 15b) with the electrodes (11a, 11b). In the case of an incorrect correspondence, the identification pieces (19a, 19b) cannot be fitted into the recesses (14a, 14b, 23a, 23b) and interfere with the upper surface of the battery (B), making assembling impossible.