PURPOSE: To obtain optimum talking state by detecting a level of noise included in transmission reception sound and transmission respectively busy tone and employing fuzzy control so as to adjust the gain of a transmission reception tone amplifier circuit depending on each obtained level and a desired gain adjustment degree to be inputted.

CONSTITUTION: A reception level extraction section 11 extracts a level of a received signal tone itself, a difference extraction section 13 extracts a change in the inputted received level signal x5 and the change is sent to a fuzzy deduction section 9 as a signal x8. A noise extraction section 12 extracts a level of noise such as a background tone included in the inputted reception signal and sends it to the fuzzy inference section 9 as a signal x6. The fuzzy inference section 9 calculates an optimum output based on a preset fuzzy rule according to the fuzzy inference and converts the result into a non-fuzzy signal and sends the gain change for transmission reception to a gain control section 15 as output signals y1, y2. Thus, the excellent talking state is always maintained corresponding to the change in the environment quickly.