To provide a data transmitter-receiver or the like that transmits/ receives data while suppressing the communication cost.

A radio wave is intermittently sent to check the presence of a response (step SA1). Whether or not a communication opposite party is found out is discriminated depending on the presence of this response (step SA2), and when the communication opposite party is found out, a communication channel with the opposite party is established (step SA3). Then an application ID is exchanged (step SA4) and whether or not the communication opposite party is an electronic mail transmitter-receiver having a similar near-by mail function to that of this electronic mail transmitter-receiver is discriminated (step SA5). When the communication opposite party is the electronic mail transmitter-receiver having the similar near-by mail function, a near-by mail software program is started (step SA6) to execute near-by mail communication processing (step SA7). Thus, the electronic mail transmitter-receiver sets that are found out can directly send/receive an electronic mail.