A positive electrode plate and a negative electrode plate are spirally wound with a separator interposed therebetween to form an electrode group 3 having a circular shape or an oval shape. Next, a center core member 2 for applying a pressure to the electrode group 3 is inserted in a winding core hole 6 of the electrode group 3. Thereafter, the electrode group 3 is deformed into a flat shape. Then, the flat electrode group 3 is accommodated in a rectangular flat armoring can 4 with a base and an opening portion of the armoring can 4 is sealed, thereby producing a rectangular flat secondary battery.