A positive electrode sheet and a negative electrode sheet are laminated and wound around a shaft core 126 while being insulated by a separator. Spreading operation plates 90P and 90N are fitted to both ends of the shaft core 126 in a winding axis direction, and operation protrusions 91A to 92B of the spreading operation plates 90P and 90N protrude from both flat end surfaces of a wound body 120. The spreading operation plates 90P and 90N are spread, so that laminate parts of a positive electrode connection part 122A and a negative electrode connection part 124A are pushed and extended outward on the flat end surfaces of the wound body 120, and V-shaped openings 120V continuous with the innermost peripheries of the wound body 120 are formed. The laminate parts are welded to positive and negative electrode current collectors 115 and 116.