Disclosed is a rubber reinforcing fiber cord in which an eco-friendly silk material is made usable for applications in an automobile pneumatic tire and the like acted upon by large loads. The rubber reinforcing fiber cord of the present invention is characterized in that: a multi-filament twisted cord formed of silk fibroin fibers having a total fineness of 1500 to 9000 dtex is covered with an adhesive agent formed of resorcin, formalin and rubber latex in order that a dip pickup thereof on the cord can become 4.0 to 8.0% per unit weight of the fibers; and the cord has an initial tensile strength not less than 3.5 cN/dtex, a high-temperature strength retention rate not less than 80%, and a post-moisture-absorption strength retention rate not less than 85%. The cord is usable in a belt reinforcing later and/or a carcass layer of a radial pneumatic tire for a passenger car.