To provide a method for treating waste catalysts in which platinum group metals are recovered from waste catalysts such as automotive catalysts and the aluminum content and magnesium content included in waste catalysts are separated as filterable precipitates in the early stage to facilitate wet treatment.

The method for treating waste catalysts comprises: a sulfuric acid leaching step of leaching the waste catalyst with dilute sulfuric acid to elute the aluminum content and magnesium content and subjecting the resulting catalyst solution to solid-liquid separation; a precipitation step of adding a calcium compound to the separated liquid to produce precipitates of gypsum sulfate; and a separation step of separating the aluminum content and magnesium content fetched in the precipitates together with the precipitates by solid-liquid separation. At the sulfuric acid leaching step, it is preferable that the waste catalyst is immersed in the dilute sulfuric acid in an airtight vessel and the resulting catalyst solution is heated to ≥100°C to perform pressure leaching.