PURPOSE: To treat a waste photographic processing liquid at a low cost by providing a means for detecting the liquid level of the waste photographic processing liquid of a float type or electrode type in a concentrating column in environment where the temp. falls to ≤70°C and washing the means for detecting the liquid level by the supplied waste liquid.

CONSTITUTION: The sludge of the concd. liquid is flushed away by the supplied waste photographic processing liquid and float and electrode surfaces are maintained always clean if the evaporating and concentrating temp. of the waste liquid are below 70°C. A means for maintaining the environment where a level sensor (LC) 64 as the means for detecting the liquid level is placed at ≤70°C includes a method of reducing the pressure in a column 1 for evaporating the concd. waste liquid to lower the b.p. to ≤7O°C, etc. The supply of the waste photographic processing liquid into the column 1 is executed by connecting a container 63 and a pump 62 with a pipe of a liquid supplying means 3 and setting the front end 61 of the pipe to the upper part of the column 1.