PURPOSE: To obtain the digital transmission system able to recognize a pushbutton signal accurately.

CONSTITUTION: A voice A/D conversion means 301 of a sender side A/D converts a voice signal in an input analog signal and a PB signal A/D conversion means 302 converts the pushbutton signal (PB signal) into a digital code. A signal discriminator 303 discriminates whether the signal is a voice signal or the PB signal, a selection means 304 adds an identifier representing a signal is a signal from a voice A/D conversion means or a signal from the PB signal A/D conversion means to the original signal depending on the result of discrimination. A receiver side uses a voice D/A converter means 601 and a PB signal D/A converter means 602 to eliminate the identifier from a received digital signal respectively and apply D/A conversion to the signal and an identifier discrimination device 603 discriminates whether the signal is the voice signal or the PB signal based on the identifier, Based on the result of discrimination, a selection means 604 selects from which of the voice D/A converter means and the PB signal D/A converter means an analog signal or outputs.