An anticaking agent for an iron and steel slag contains (A) at least one member selected from the group consisting of a phosphonic acid derivative and a salt thereof and (B) at least one member selected from the group consisting of an aliphatic hydroxycarboxylic acid and a salt thereof, at a mixing ratio (A)/(B) of the at least one member selected from the group consisting of a phosphonic acid derivative and a salt thereof (A) to the at least one member selected from the group consisting of an aliphatic hydroxycarboxylic acid and a salt thereof (B) of from 95/5 to 5/95 by mass, and the anticaking agent for an iron and steel slag may further contain (C) a sugar alcohol, thereby providing further high synergistic effect on anticaking capability.