A data communication system includes a host device, a reader/writer device and a plurality of contactless identification tags. The contactless identification tag includes a data reception section, a data transmission section, a data control section, a data storage section, a driving electric current generation section and a coil antenna. In response to a command from the reader/writer device, the data control section generates a random number, and sets a time slot number based on the random number and a unique identification number stored in advance in the data storage section.