To provide a lead acid battery reducing damages of a separator in contact with a positive electrode plate, and having overcharge resistance and vibration resistance.

A lead acid battery includes a positive electrode plate and a negative electrode plate stored in a bag-shaped separator made of polyolefin. The bag-shaped separator has a projecting rib, whose apex is in contact with the positive electrode plate, on the surface facing the positive electrode plate, and has a recessed part on the internal surface corresponding to the disposed position of the projecting rib. The depth of the recessed part is in the range of 0.35-0.55 of the base thickness of the bag-shaped separator. The open width of the recessed part is preferably in the range of 1.6-2.4 of the base thickness of the bag-shaped separator.