A compound for use as a cross-linking agent represented by the following formula (1) wherein R1 is —SO2—, —O—, —CO—, an optionally substituted alkylene group, a group represented by the following formula (2) or a single bond; and two As are each independently an aromatic ring group containing a 5- or 6-membered aromatic ring and optionally containing a substituent, where the aromatic ring group contains a substituent in the case where the aromatic ring group is a monocyclic aromatic ring group containing a 6-membered aromatic hydrocarbon ring. Also disclosed is a composition containing the cross-linking agent.