A film cartridge assembling apparatus including one or more assembly chucks each including two die elements formed by combining partial cylindrical inner surfaces thereof together. The circumference of a resulting cylinder being partially cut to form an opening. One of the die elements includes a member having a flat surface extending from an end of the cylindrically-shaped inner surface towards the opening. The member is adjustably mounted to the one of the die elements. The assembly chuck is able to assume a closed state in which the two inner surfaces of the two die elements are matched to each other to form a cylindrical inner surface and an opened state in which ends of the two die elements substantially parallel to the center axis of the cylindrical inner surface are spaced apart from each other.