A processing solution useful for the development of or halftone dot reduction of a metal image-forming material comprising a support, a thin metallic layer composed mainly of aluminum on the support and a photosensitive resin layer on the metallic layer, with the processing solution comprising (1) an alkaline solution, such as an aqueous solution of sodium hydroxide or potassium hydroxide, and (2) at least one compound of the formula (I) or (II):m(brO.sub.3).sub.p (I)m.sub.m (IO.sub.n).sub.p (II)wherein M represents a hydrogen atom or a metal having a valence of 1 to 3, m represents 1, 2, 3, or 5, n represents 3, 4, or 6, and p represents 1, 2 or 3, which solution lends itself to safe storage and handling, and permits rapid and uniform etching of the metallic layer of the metal image-forming material without bubble formation occurring.