PURPOSE: To obtain a small-sized dry type developing device free from gas leakage by inclining the opening surface of a V-shaped tray for supplying a developing solution, fitting a guide plate on the opening surface so as to be parallel with the opening surface and fixing a sealing part on the end part of the higher side of the guide plate.

CONSTITUTION: The surface of an opening 1a of a tray 1 with a V-shaped section is mounted so as to be inclined and a developing solution 3 (ammonia water) is supplied from a feeding pipe 2 to the tray 1. Plural wires 6 are fitted to the opening 1a, a guide plate 7 is arranged so as to be parallel with the surface of the opening 1a and a sealing member 9 is fixed on the end part 7a of the higher side of the guide plate 7 to seal the upper flange part 5a of the tray 1. Ammonia water 3 is heated and evapolated by a heater 4 and the surface of the opening 1a is filled with gaseous NH3. The gaseous NH3 is protected from being leaked by the sealing member 9. When photosensitive paper 8 is carried from the lower part as shown by an arrow A, the coupler components contained in the photosensitive paper 8 are reacted with the gaseous NH3, attaining diazo wet development.