Abstract: An improved developing apparatus for diazo-type photocopy machines, which is adapted to prevent external leakage of gaseous ammonia developing medium to the surrounding environment is provided. The apparatus comprises a developing chamber having transport means for advancing exposed diazo-type sensitized paper therethrough, means for introducing and circulating gaseous ammonia within the chamber to develop an image on the sensitized paper; and a vacuum pump for evacuating said ammonia from the developing chamber. As an additional feature of the invention, vacuum surge means are provided to maintain the below atmospheric pressure within the developing chamber at a relatively constant level to ensure uniform development of the sensitized paper.