PURPOSE: To provide a plasticizer for synthetic resins, which can impart excellent plasticity to synthetic resins and does not cause any troubles on hygiene such as carcinogenicity, consisting of a chemically modified polybutene as a main ingredient.

CONSTITUTION: A plasticizer consists of a chemically modified polybutene as a main ingredient. Particularly pref. polybutene to be modified is liquid polybutene having a number-average MW of 350W2,500 obtd. by copolymerizing isobutene with a small amount of n-butene. As said chemically modified polybutene, epoxidized or aromatized polybutene is preferred. The plasticizer may be composed of a chemically modified polybutene alone. If desired, other plasticizer component or other additives may be incorporated therein.