A communication terminal device, a communication system, a communication method, and a program which allow efficient communication processing via multicast communication, along with power saving. The communication terminal devices includes a wireless communication unit comprising: a multicast ACK generating unit which makes it possible to transmit packets having an ACK function; a multicast ACK analysis unit which analyzes received packet data for ACK information; wireless transmission and reception units which perform modulation and demodulation processing; and a central control unit and a timing control unit which control these components. The ACK function is thus implemented by multicast packets in multicast communication. This makes it possible to check if its own transmission data is received by other stations (communication terminals).