To extremely retard the degree of progression of deterioration by contact of a processing liquid with air by eliminating such trouble as an increase of weight by embedding of solids etc., and reducing the volume in the spaces communicating with the outside of the device existing in the upper spaces of a treating vessel.

The large space in the upper part of a developing tank 24 is isolated by a first box body 200 and a second box body 206 to suppress the circulation of the air between the space and the outside of the device exclusive of the transporting route of a PS plate 12 which is the minimum required space, by which the oxidation of the developer by contact with the air may be suppressed. In forming the first box body 200 and the second box body 206, air packing cells 212 are packed therein in a hexagonal close-packed state, by which the need for such precise alignment and filling as to completely prohibit the flow of the interior and exterior air is eliminated and the assembly workability may be improved.