The present invention provides a method and an apparatus for treating boron-containing waste, which prevent clogging of a waste liquid line, achieve an increase in feeding of a waste liquid to a dryer, make in-drum mixing feasible, and increase the strength of a cement solidification product. A method for treating a boron-containing waste liquid comprises the step of adding an alkali metal element compound and an alkali earth metal element compound to the boron-containing waste liquid, and the step of then drying and powdering the boron-containing waste liquid, wherein the temperature of the boron-containing waste liquid during a period from addition of either the alkali metal element compound or the alkali earth metal element compound until the drying and powdering is maintained at not lower than the precipitation temperature of a compound containing boron and an alkali metal and a compound containing boron and an alkali earth metal.