This device is designed so that although voice of the opposite party will be amplified when telephone 1 on the side of the device is first placed on-hook during a telephone conversation, amplification will not occur if the opposite party places his or her phone on-hook first during a telephone conversation or if the user places telephone 1 on-hook before the opposite party picks up the telephone. The structure, which is shown in FIG. 1 such that during a telephone conversation between telephone 1 and telephone 12, when telephone 1 is placed on-hook first, the on-hook status is detected by means of the photocouplers PC-1 or PC-2, and the voice of the opposite party at telephone 12 is amplified by means of contacts y1-1 and y1-2, which are in the make position, line transformer 2, amplifier 3 and speaker 4. Further, when the opposite party's telephone 12 is placed on-hook first, the voice of the opposite party will not be amplified because a reversal of polarity and a brief disconnection on telephone line 13 are detected by means of photocouplers PC-1 and PC-2.