Abstract: A data collection system performs data transmission from a portable information collection-transmission unit 1 to a central data reception unit 7 over a telephone line 6. The information collection-transmission unit 1 has a terminal 2 which temporarily stores data and a unidirectional acoustic coupler 3 which converts the data held in said terminal 2 into binary codes of frequencies used for the transmission on the telephone line, and it sends the data over the telephone line 6. The data reception unit 7 has a modem 8 which converts the data signal on the telephone line into a usable digital data signal and a work station 9 which stores the output of the modem, and it is further provided with a voice synthesizer 10 which outputs a prescribed low-frequency audible signal to the telephone line 6 in response to the instruction from the work station 9.