A transmission device in which a bus of a central processing unit is used to synchronize timing signals between units, thereby restraining enlargement in scale of wiring. A reference signal generator generates a reference signal. A reference signal receiver is mounted on a unit set as an active or standby unit and receives the reference signal. A timing signal generator divides the frequency of the received reference signal by means of a frequency divider/counter to generate a timing signal. A count holder holds the count value of the frequency divider/counter. The bus connects the units and the central processing unit. A count receiver receives, via the bus, the count value from the count holder of the active unit. A count updater updates the count value of the frequency divider/counter to the count value received by the count receiver.