A method for stabilizing an aryl boron compound of formula (1): wherein Ar1, Ar2, and Ar3 are aryl groups, including mixing a compound which has higher affinity for the aryl boron compound than water, as a stabilizer, in a solution or slurry containing the aryl boron compound in a hydrocarbon solvent; and a stabilized composition obtained thereby. The stabilizing method and the stabilized composition of the present invention make it possible to store aryl boron compounds stably over a long term, while keeping their high purity, in the form of forming a substantially homogeneous system in the case of a solution or in the form of forming a substantially homogeneous system by heating in the case of slurry.