1. A synchronization system among processors, comprising: a plurality of processors; an information sharing system for holding information shared with said processors; means responsive to an instruction from a certain processor, for outputting a synchronization request signal for requesting synchronization of processors associated with said certain processor; and means for generating and outputting a synchronization completion signal when the synchronization request signals for all of said associated processors to be synchronized has been outputted, wherein there are provided a synchronization instruction having a function of generating from said certain processor an instruction for making said synchronization request signal outputting means generate said synchronization request signal, and means for performing local synchronization in such a manner that when said certain processor accesses said information sharing system after said certain processor has executed said synchronization instruction, an access to said information sharing system by said certain processor is not permitted and processing of said certain processor is suspended in a case where said synchronization completion signal corresponding to said synchronization instruction has not been generated, until said synchronization completion signal is generated, and on the other hand an access to said information sharing system by said certain processor is permitted and said certain processor can continue a next processing in a case where said synchronization completion signal corresponding to said synchronization instruction has been generated.