To treat a waste oil based on an organic ester compound inexpensively or safely by using a finely powdered waste concrete material based on a cement hydrate as the effective component.

The effective component of the waste oil treating agent is a cement-hydrate-based finely powdered (about 300 μm or below) waste concrete produced by crushing a waste concrete material using a portland cement such as high-early-strength portland cement or normal portland cement and recovering the aggregates, reinforcements, etc., from the crushed product. When the treating agent is brought into contact with a waste oil based on an organic ester compound by e.g. immersion, the waste oil can be safely and inexpensively treated because it changes from a liquid to a paste or a solid because the organic ester compound is hydrolyzed into the corresponding carboxylic acid, which then reacts with the calcium of the cement hydrate. The waste oil as the object to be treated is desirably e.g. a waste solvent based on ethyl acetate, methyl acetate, or the like.