PURPOSE: To eliminate distortion of voice due to echo by measuring an echo time before a voice transmission/reception device is in use so as to adjust the time constant of a rectifier circuit at the reception side in the voice transmission/reception device provided with a side tone preventing circuit.

CONSTITUTION: The echo time of an ordinary room is constant when the time is settled once. Thus, when the device is set, the said echo time is measured to control the discharge time of a rectifier circuit 23. That is, a signal generator 20 is connected to the input of a speaker 7 to output a waveform shown in figure (a). As a result, a waveform shown in figure (b) appears at the output side of an amplifier 2 due to the effect of the echo via a microphone 1. A detector 21 detects this waveform to measure a time (t) required for the voltage level of the said waveform attenuating from a voltage V1 into a voltage V2. When the time (t) is large, the echo time is longer and when small, the echo time is shorter. Thus, the discharge time constant of the rectifier circuit 23 is controlled when the said time (t) is within a range and the time constant is not controlled when the time (t) is at the outside of the said range.