PURPOSE: To multiplex and transmit data different in speed, by transmitting plural data, which are synchronized with an optional clock frequency with a maximum frequency limited, to a transmission line in time division multiplexing and by separating synchronizing data in the receiving side by the sane clock frequency.

CONSTITUTION: Plural transmitted data Di∼Dn are synchronized with data clocks (a), and data clocks (a) are optional with a maximum frequency limited and are measured by a frequency measuring means 1 and are encoded by an encoding circuit 2 and are multiplexed as clock information Ao∼An togehter with other data D1∼Dn by a multiplexing circuit 3 and are transmitted. A clock generating circuit 4 generates constant clocks (f) considerably quicker than clocks (a). Multiplexed signals are separated through a transmission line by a separating circuit 8 of a receiving device, and transmission clocks (f) are equal to those in the transmission side, and data D1∼Dn and encoded clock information AO∼AN are obtained as the output of the separating circuit 8, and clock information AO∼AN are given to a regenerating means 10 to regenerate data clocks (m), and data is outputted from a means 11 by them.