The present invention addresses the difference in the basic concept of the communication means between a PoC system, which is a half-duplex communication system, and a general telephony, which is a full-duplex communication system, providing smooth communications. A communication system is provided with a communication controller which performs interconversion between communication control signal used in a telephone network and a communication control signal used on a PoC (Push-To-Talk over Cellular) system and a voice signal processor performing interconversion between a full-duplex voice signal used in the telephone network and a half-duplex voice signal used in the PoC system.