This invention provides a softening agent composition obtained by blending and heat treating (a) 100 parts by weight of at least one non-aromatic hydrocarbon softening agent for rubber; (b) 0.1 to 10 parts by weight of an organic peroxide; and (c) 0.1 to 50 parts by weight of a crosslinking aid, with the proviso that the weight ratio of component (b) to component (c) is 1 or less. The softening agent composition is useful and very easy to handle in the production of a thermoplastic resin composition.