PURPOSE: To provide an economical and quick communication system provided between the processors of an electronic unit, which can perform almost random access which can be compared by a master processor.

CONSTITUTION: This system is provided with a first data communication bus 1, second serial signal bus 2 being the sheet of a synchronizing signal frame constituted time intervals IT assigned to those processors, and means for synchronizing the time intervals in those processors at the time of mutually connecting (n) pieces of slave processors P1, P2,..., Pn with a master processor Pn+1. This system is provided with a first comparing means, and the periodic appearance of the time interval IT assigned to each processor is confirmed in each processor, and also the periodic appearance of each time interval IT is confirmed in the master processor by a second comparing means.