To provide a tabular secondary battery with excellent shape stability and high safety, which prevents battery breakage in the drastic increase in internal pressure of the battery.

A tabular secondary battery according to the present invention includes: a flat electrode group 11; an electrolyte; a tabular battery case 1 for housing the electrode group 11 and the electrolyte; and a support post 3 disposed penetrating the electrode group 11 in a thickness direction. The support post 3 restricts the deformation of both main surfaces 4 of the battery case 1 in a manner that both ends of the support post 3 are bonded to both main surfaces 4 of the battery case 1. The electrode group 11 is formed by stacking or winding in a flat manner a positive electrode plate (first electrode 12 or second electrode 13), a negative electrode plate (second electrode 13 or first electrode 12), and a separator 14 disposed between the positive electrode plate and the negative electrode plate. The support post 3 is provided so that the restriction on the deformation of both main surfaces 4 of the battery case 1 by the post 3 is eliminated when the internal pressure of the case 1 has exceeded predetermined pressure.