Abstract: A synchronization system for a key telephone system, in which information signals are transmitted in a time-division manner between a key service unit and each key telephone set. The information signal is formed by a control signal, a first signal transmitted from the key service unit to the key telephone set and a second signal from the key telephone set to the key service unit. The control signal is formed by a start signal of one-pulse and a variable synchronizing code, which is placed in a time slot following the start signal and which is formed by a plurality of pulses assuming one of predetermined consecutive values in a predetermined order for each transmission frame period. The control signal is transmitted from the key service unit to each key telephone set for each frame period in synchronism with a first counter provided at the key service unit for developing standard of transmission timings.