PURPOSE: To secure a multiplexed line with many channel numbers and to utilize effectively a frequency band width or a time division band width by providing a standard talking band width and a narrow talking band width of channels.

CONSTITUTION: Speeches 2-17 having a connection request are assigned sequentially to channels ch1-ch3 having a frequency band width f with a high quality channel at first. Then the allocation of lines is changed to channels ch4-ch7 having a narrower band width than that of the band width f being low quality channels after a prescribed time elapses. Moreover, in the case of the time division system, the channels ch1-ch3 are used for high quality channels, a time slot t is allocated, channels ch4-ch7 are used for low quality channels and a time slot ts less than the time slot t is assigned. The line allocation is moved from high quality channels to low quality channels for the speech after a prescribed time in this way to secure lines with many channel numbers and the frequency band width or the time division time band width is effectively utilized.