A roadside-vehicle cooperative illumination system in which a headlight of a vehicle and a road illumination lamp can be controlled cooperatively through communications between an in-vehicle apparatus and a roadside apparatus. For example, when power consumption of the headlight has to be reduced, an in-vehicle apparatus side arbiter transmits a request to the roadside apparatus for increase illumination of the road illumination lamp. A roadside apparatus side arbiter determines whether or not to accept the request from the in-vehicle apparatus, and a roadside apparatus side controller controls the road illumination lamp according to the determination of the roadside apparatus side arbiter. This enables the road illumination lamp to be controlled cooperatively so that illumination of the road illumination lamp can compensate for deficiencies in illumination of the headlight, which leads to enhancement of vehicle driver's convenience and ensuring of safety.