PURPOSE: To allow the outputting of information at the time suitable for sure transmission of the information by detecting a sound volume, comparing the sound volume with arbitrary threshold and controlling the information so as to change the magnitude of the information to be outputted according to the compared result.

CONSTITUTION: The ambient noise inputted from a microphone 1 is inputted to an average value detecting section 2. This average value is read into a CPU 3 and is stored in a RAM 5. The threshold for judging whether the sound volume of the sound outputted from the average value is amplified or not is previously determined. Whether the average value of the inputted noise volume is larger than the preset threshold 1 or not is judged by a CPU 3 and an instruction is so given to an amplification factor controller 6 as to increase the amplification rate of an amplifier device 7 when judgment is made that the above- mentioned average value is larger. The disappearing of the outputted sound by the ambient noise is obviated in this way and the sure transmission is executed.