To provide a method for manufacturing an endless metallic thin film in which an electrocasting film can be easily demolded from a cylindrical metal mother die even if the inner diameter of the electrocasting film becomes 30 mm or less.

An endless metallic thin film (electrocasting film) 25 is formed by electrocasting on the outer periphery of a cylindrical mother die 10, which is constituted by (a) a body part 1 provided with a small-diameter part 1a in which the diameter of the circumference is gradually reduced from the vicinity of the outside lower end to the lower end and (b) a lower end part 2 provided with a skirt-shaped part 2a having a recessed part 2b in which the small-diameter part 1a is detachably fitted and the diameter of the circumference is gradually increased from the vicinity of the inside lower end to the lower end at the upper part and a brim-like flange 2e at the lower part. Then, the endless metallic thin film 25 is lifted and the lower end part 2 of the cylindrical metal mother die 10 is removed so as to pull out to form a space in the inside vicinity of the lower end part 2 of the cylindrical mother die 10 and subsequently gas is blown into the space under high pressure to demold the endless metallic thin film 25.