PURPOSE: To transmit quickly the generation of an ineffective data to a receiving side DTE owing to the generation of a gap in a data between a receiving side STDM and the DTE caused by the generation of underrun, and to prevent a buffer of the DTE filled in with the ineffective data.

CONSTITUTION: A statistical time division multiplexer STDM5' transmits a data 8 added with an abort pattern 9 to a data terminal device DTE1' in succession to the data 8 during transmission when the underrun is generated and informs that the data transmitted already is ineffective. That is, when it is detected that the transmission data buffer 14 is idle before a transmission control section 15 of the STDM5' extracts a character representing the end of data from the transmission buffer 14, a switching section 13 is switched to an abort pattern generating section 12 to transmits an abort pattern to the output and informs it to the DTE1' that the data transmitted already is ineffective.