To easily realize avoiding of congestion, before its occurrence by detecting the congestion state early.

A relay 4b of an IP network 3, which connects a plurality of terminals 1 and 2 adds a congestion value 19, either to 1 or to 0, to a voice packet 11 to be transmitted to the terminal 1. The terminal 1 comprises a setting part 29 which sets a call, and a detecting part which receives a voice packet with congestion value 11' for detecting congestion state, based on the congestion value 19 of the voice packet with congestion value 11'. The setting part 29 selectively decides a coding method, corresponding to the congestion state. Since a congestion value is inserted, while the voice packet is transmitted, the congestion value is easily transmitted and increase of traffic, when the congestion value is transmitted is suppressed to bare minimum, and congestion is avoided before it takes place, and degradation of voice quality is avoided.