Provided are a case assembly, a prismatic secondary battery, and a method of fabricating the prismatic secondary battery. The prismatic secondary battery includes: an electrode assembly including a first electrode plate, a second electrode plate, and a separator between the first and second electrode plates; a case formed of a first material, wherein an electrode assembly accommodation cavity is formed in the case to accommodate the electrode assembly, and an opening through which the electrode assembly is to be inserted is formed in a portion of the case; and a cap plate sealing the opening. A gas discharge hole is formed in the case to discharge gas generated during a pre-charging operation. The gas discharge hole is sealed with a gas seal formed of a second material.