Abstract: A high efficiency speech coding apparatus which can eliminate repetitions of high efficiency speech coding and decoding processes of a digital speech signal upon inter-office transit switching without employing a special exchange and a transit switching system which can prevent degradation in quality of speech information and increase in encode and decode processing delay upon transit switching are disclosed. The apparatus repetitively inserts a synchronizing signal into an output speech signal to an exchange and supervises an input speech signal from the exchange to detect a synchronizing signal in order to determine whether the destination of connection of the exchange is a subscriber line or another trunk circuit connected by way of another high efficiency coding apparatus.