Disclosed herein is a prismatic battery having an electrode assembly mounted in a prismatic battery case, wherein an electrolyte injection hole formed in a base plate mounted to an open upper end of the battery case is constructed such that an inside upper end of the electrolyte injection hole is formed in a downward taper (incline) structure, and a metal ball is forcibly inserted into the electrolyte injection hole such that the metal ball is plastically deformed to seal the electrolyte injection hole. The prismatic battery according to the present invention provides high coupling force between the metal ball and the electrolyte injection hole due to the plastic deformation and forced insertion of the metal ball and high sealability of the electrolyte injection hole. Also, the formation of a groove at the upper end of the forcibly inserted metal ball and the generation of cracks at the interface between the metal ball and the inside surface of the electrolyte injection hole are effectively prevented. Consequently, the present invention has the effect of accomplishing high electrolyte sealability.