A developing machine having a high durability at a high temperature even in the presence of a high concentration of the developing agent is provided. The developing machine has means for keeping the temperature of a color developer having a concentration of an aromatic primary amine developing agent of 0.15 to 0.50 mol/l in the range of 40.degree. to 50.degree. C., and the material for at least one of the developing tanks and the racks is a resin selected from the group consisting of polyphenylene sulfide resin, polyphenylene oxide resin, polymethylpentene resin, polyether ether ketone resin, polyalkylene terephthalate resin, polyether imide resin, polyether sulfone resin and polysulfone resin.