A lead material (5) for a negative electrode is made of a clad material (50) including a Cu layer (51) made of Cu or a Cu alloy and Ni layers (52, 53) each made of Ni or a Ni alloy. The Ni layers are respectively bonded to opposite surfaces of the Cu layer. The Ni layers each include a surface (52b, 53b) not bonded to the Cu layer, the surface including an oxide film (52c, 53c) with a thickness of 30 nm or less.