To provide radio data transmitting devices capable of performing a communication at the optimum transmission speed even if the radio data transmitting devices are located at a mutually long separation.

This device is provided with an electric field intensity decision processing part 20, a transmission unit decision processing part 21, an electric field intensity/transmission unit correspondence table 22 and a transmission unit change processing part 23, the part 20 reads electric field intensity ei obtained by the baseband signal processing part 13 of a radio terminal 1 via serial line 3, and decides whether the intensity ei exists within a certain fixed range of the preset initial value of the electric field intensity, and the part 21 reads the decision results of the part 20, newly selects any transmission unit hi in the table 22 and makes contents data a frame in the transmission unit, in the case of deciding that the intensity ei exists outside the fixed range, and transmits the contents data to baseband signal processing 13 of the terminal 1 via a serial interface 3.