PURPOSE: To maintain the preservation stability of a processing soln. without decreasing the capacity by providing a treating tank with the opening area ratios of the respective treating tanks of an automatic developing machine regulated below a specified value when the opening ratio is the ratio of the opening area of the treating tank to the capacity of the treating tank.

CONSTITUTION: The deterioration of a processing soln. is accelerated as the opening area S of a solid processing agent inlet 20 is increased, and hence the opening area S is preferably made as small as possible. When the area of the inlet 20 opened to the upper of a main body 101 is Scm2, the total amt. of the processing soln. in a treating tank 1A is Vl, and the opening area ratio N is Scm2/Vl, the ratio N is controlled to ≤12cm2/l to produce satisfactory results. Meanwhile, when a tablet J as the solid processing agent is supplied to the inlet 20 in an automatic developing machine, the opening area S is controlled to the area S with the area according to the condition of the opening area ratio N controlled to ≤12cm2/l added with the area for passing the tablet J to produce a good result.