NOVELTY - A bipolar lithium ion secondary battery consists of a bipolar electrode (4) having a positive-electrode active-material layer (2) formed on one surface of a stainless steel foil (1) collector having a roughened surface whose crest interval/surface roughness ratio is 5 or less, and a negative-electrode active-material layer (3) formed on the other surface, laminated through an electrolyte layer (5). The foil is subjected to alternation electrolytic etching in aqueous ferric chloride, and a spherical pit is formed without a gap.    USE - Bipolar lithium ion secondary battery is used for electric vehicles, hybrid vehicles, and mobile telephones.    ADVANTAGE - The bipolar lithium ion secondary battery has excellent battery characteristics, such as output characteristics, cycle characteristics, long-term stability, and safety. Even when a stainless steel foil having corrosion resistance in positive electrode and negative electrode environment is used as a collector of the bipolar electrode, the resistance of the entire battery can be reduced. The need for cladding of stainless steel foil without pinholes, as in the case of aluminum or copper foil, is eliminated. A lightweight and compact secondary battery having large area can be manufactured economically. Disorder of current distribution accompanying expansion and contraction of active-material layer during charging/discharging, is prevented.    DETAILED DESCRIPTION - A bipolar lithium ion secondary battery consists of a bipolar electrode having a positive-electrode active-material layer formed on one surface of a stainless steel foil collector having a roughened surface whose crest interval/surface roughness ratio is 5 or less, and a negative-electrode active-material layer formed on the other surface, laminated through an electrolyte layer. The foil is subjected to alternation electrolytic etching in aqueous ferric chloride, and a spherical pit is formed without a gap. When the surface roughness of the foil is measured with respect to a surface area of 50 mu m rectangle with a scanning-type confocal laser scanning microscope with a resolution of 0.01 mu m, the surface roughness is 0.1-2 mu m.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic cross-sectional view of the bipolar lithium ion secondary battery.    Stainless steel foil (1)    Positive-electrode active-material layer (2)    Negative-electrode active-material layer (3)    Bipolar electrode (4)    Electrolyte layer (5)