The present invention has a main object to provide a modifier for hydrocarbon polymer that improves a coatability of a surface of a hydrocarbon polymer molding. To attain the object, the present invention provides a modifier for hydrocarbon polymer which contains, as an active component, a modified cyclic structure-containing conjugated diene polymer wherein an amount of an α,β-ethylenically unsaturated carboxylic acid compound bonded by addition is from 0.1 to 20% by weight and a cyclization ratio is from 30 to 95%; and provides a hydrocarbon polymer composition comprising the modifier for hydrocarbon polymer and a hydrocarbon polymer; and provides a molding by molding the hydrocarbon polymer composition.
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