Provided are a reinforcing cord using a reinforcing fiber treating agent for improving the performance, particularly oil resistance, of rubber products, and a highly oil-resistant rubber product. By using a reinforcing fiber treating agent that has an alpha,beta-ethylenically unsaturated nitrile monomer unit content of 30-55 wt %, an acid group-containing alpha,beta-ethylenically unsaturated monomer unit content of 3-20 wt %, an iodine value of 120 or less and a tetrahydrofuran insoluble fraction of 30 wt % or more, the oil resistance of the reinforcing cord and thus the oil resistance of rubber products using the reinforcing cord are improved.