PURPOSE: To make use of all parts of a thin plate, and prevent any waste of the material by cutting the thin plate from its center to the end in such a manner that lags having the same length as an expanded part of the plate are formed, before the plate is subjected to expanding work.

CONSTITUTION: Lags 2 and 2' are provided at an alternating side of a thin plate 1 made of lead or a lead alloy by cutting the plate 1 from the center to each end of the plate 1 by press work. The lags 2 and 2' are bent from the ends of the cuts toward an expanding die in order to prevent the lags 2 and 2' from being expanded. After that, the plate 1 is expanded from its both sides. The plate 1 expanded is cut so that it is separated at the center of a frame bone 5, thus a base body being obtained. The lags 2 and 2' bent are restored to their former conditions before paste is packed into the base bodies.