Flat battery

A flat battery comprising:

• an exterior can including a bottom and a cylindrical side wall;
• a seal can including a top-face portion and a peripheral wall and facing the exterior can;
• and
• a gasket positioned between the cylindrical side wall and the peripheral wall,
• wherein:
• the peripheral wall includes:
• a base-end portion adjacent to the top-face portion;
• a cylindrical portion having an outer diameter larger than an outer diameter of the base-end portion and having an extreme edge facing the bottom of the exterior can;
• and a stepped portion located between the base-end portion and the cylindrical portion;
• the cylindrical portion is inclined radially outwardly going from an end adjacent to the stepped portion toward the extreme edge of the cylindrical portion and has an angle of inclination of 0.5° to 4° relative to an axial direction;
• and
• the base-end portion is inclined radially inwardly going from an end adjacent to the top-face portion toward an end adjacent to the stepped portion and has an angle of inclination of 0.5° to 5° relative to the axial direction.