PURPOSE: To reduce the probability of occurrence of disabled communication between communication equipments due to a fault of a clock supply route by supplying a synchronizing clock with high priority to each communication equipment as a reference clock in response to the state of a communication network.
CONSTITUTION: Clock generating sources 11a, 11b generate a clock supplied to a communication equipment 101 and a clock generating source 11c generates a clock fed to a communication equipment 102. In this case, different priority is given to clocks generated from generating sources 11a-11c and an added number in response to the number of communication equipments transmitted after the occurrence is added to each clock. Then a clock with the highest priority and added with the least added number in the number of communication equipments with the clock transmitted therethrough is selected among plural clocks supplied to the equipments 101-103, the equipment is operated synchronously therewith and the number is incremented by one and the result is outputted. Thus, even in the presence of a fault in the supply route of a communication equipment, synchronization is takes with other communication equipment, normal communication is made available and the communication disabled state is reduced.