PURPOSE: To prevent any adhesion of metallic lithium to a lithium-sheet-stamping machine which might be caused when a lithium sheet used as a negative active material is formed by stamping by using a plastic material having a small water absorption rate to form the punch and the die of the stamping machine.

CONSTITUTION: When forming lithium sheets used as a negative active material for a lithium battery by stamping, at least one of the die 1 and the punch 2 of a stamping machine is formed by a plastic material having a water absorption rate of 0.03% or below represented by polyethylene, polypropylene and polytetrafluoroethylene. As a result, any adhesion of metallic lithium to the outer peripheral surface of the punch 2 or to the inner peripheral surface of the die 1 which might be caused due to a minute amount of water contained in the materials of the punch 2 and the die 1 can be prevented. Accordingly, the efficiency of continuous process manufacturing is improved and the dimensions, the shape and the quality of the metallic lithium sheet produced can also be improved.