A facsimile device for transmitting image data, using a computer communication network such as the Internet. A transmitting side facsimile device FAX1a determines whether the Internet NTW or the public switched telephone network PSTN is used, by confirming whether information showing to use NTW is attached to the facsimile number of the destination facsimile device FAX2. If NTW is used, image data is transmitted to the gateway device GTW1 and the facsimile number of FAX2. Users can easily select a requested communication network. FAX1a discriminates the beginning number attached in the beginning of the destination number input by users and confirms whether the internal network K, PSTN or NTW is used in sending image data. If NTW is used, the IP address of the nearest gateway device GTW in FAX2 is acquired and image data and the destination number is transmitted by designating the IP address as a destination.