To avoid the breakage of a buffer and the generation of noises and the discontinuation of decoding of the compressed signals of the video and audio signals, etc., which are caused by the buffer breakage by displacing the oscillation frequency of a receiving system clock by the output of a monitoring circuit and securing the average synchronization between the transmitting and receiving system clocks.

The compressed audio data (f) is written in a audio data buffer 304 via a receiving terminal 3. The data occupation rate of a data buffer memory storing the compressed data (e, f) that are essential to the terminal 3 is monitored by a buffer occupation rate monitoring device 311 through the window of the prescribed width. Then the device 311 decides one of both sides of the window where the state of the data buffer memory gets out of its position. The oscillation frequency of a receiving clock is simply controlled by the output of the device 311, and the average synchronization is secured between a transmitting system clock and a receiving read system clock. In such a constitution, it is possible to attain a compressed signal transmission system of high reliability where a complicated PLL circuit can be omitted even at the receiving side.