Abstract: An interprocessor communication system is used in an information processing system having a plurality of processor units. Those processor units are classified into a first group comprising control processor and IO processor units and into a second group comprising a plurality of execution processor units. In order to enable to start communication between two of the execution processors when one unit in the first group is performing communication with another one of the plurality of processor units, the interprocessor communication system comprises first through third locking circuits for issuing a first through third permission and then locking further issue of the first through third permissions, respectively. Two units in the first group communicate with each other after acquiring the first permission. Two units in the second group communicate with each other after acquiring the second permission. One unit in the first group communicates with one unit in second group after acquiring the third permission.