a multiprocessor system includes a plurality of central processing units , which have a main storage in common , and a key storage for storing therein control information for storage protection of , reference to , and change in the main storage . each cpu is provided with the key storage , the cpus are connected by interface lines so as to form a ring - like combination , a cpu in which a key access request is generated , carries out the key processing for its own key storage and supplies the interface line with an address , data and others which are contained in the key access request , and another cpu receives the address , data and others to perform the key processing for its own key storage . a signal for determining the priority among key access requests simultaneously generated in a plurality of central processing units circulates through the cpus via the interface lines , one of the cpus having generated the key access requests catches the priority determining signal to make its own key access request valid , and prevents the circulation of the priority determining signal for a time till the key processing based upon the valid request has been completed .