Storage battery

A storage battery (1) includes a battery casing (3), a cover (2) closing an opening of said battery casing (3), and a thinner see-through window portion (4) provided on a sidewall of battery casing (3) for visual recognition, from the outside, of a level of electrolyte contained in battery casing (3). A rib (5) for reinforcement is provided at see-through window portion (4).