A rectangular air battery includes: a first case having a bottom with air holes; a second case; and an insulating gasket with a substantially U-shaped cross-section. The first case and the second case are joined such that the opening of the second case faces the bottom of the first case, and are sealed by crimping the opening edge of the sidewalls of the first case onto the outer face of the second case, with the outer sidewalls of the insulating gasket interposed therebetween. A rib is provided on the bottom of the first case so as to protrude inward inside the first case. The rib extends along the four sidewalls of the first case and supports the inner lower end of the inner sidewalls of the insulating gasket, with at least a separator interposed therebetween.