The present invention relates to a coupling agent for rubber/carbon black, containing a sulfide compound represented by the following chemical formula (I): (in the formula, each A represents O, S, NH or NR; each R represents a linear or branched alkyl or alkenyl group having 1 to 6 carbon atoms, or a cyclic alkyl or alkenyl group having 3 to 6 carbon atoms, and when A is NR, the plurality of R's on a condensed aromatic heterocyclic ring may be the same as or different from each other; n represents an integer from 1 to 6; and x represents an integer from 1 to 4).