PURPOSE: To prevent the sticking of sludge to an inwall or a heater ant to improve operability for extracting a solid material by applying spray drying as a concentrating method for processed waste liquid and using a belt carrier for extracting a concentrated material, i.e. a sprayed/dried substance, from an evapolating oven.

CONSTITUTION: The temperature of processed waste liquid to be supplied to the evapolating oven 2 is set up to 30 to 80°C, preferably 50 to 80°C, and a heater 12b, a scraping spatula 13 for scraping a sprayed/dried substance from a belt 12a of the belt carrier 12 and a sludge tank 14 for storing scraped materials are provided on the inside of the belt 12a. When the processed waste liquid of a non-silver salt photosensitive material is sprayed in the oven 2 in a pressure reduced condition and a solvent is evapolated, the waste liquid is separated into the solvent, solute or the concentrated material of the processed waste liquid and the solute or the concentrated substance is stuck to the belt 12a and recovered. Thereby, the sticking of viscous sludge to the inside of the oven 2 can be prevented and the solid material can easily be extracted from the inside of the oven 2.