A digital data processing circuit includes: an output unit configured to output to an audio signal processing circuit change data for changing a receiving frequency of a FM receiving device as a first frequency to a second frequency in response to an instruction signal providing an instruction to change the receiving frequency to the second frequency, the audio signal processing circuit being a circuit configured to modulate a carrier wave having the first frequency corresponding to setting data with a modulation signal corresponding to an audio signal to be reproduced by the FM receiving device and to the change data, and transmit the modulated carrier wave to the FM receiving device; and a setting unit configured to set the setting data so as to change a frequency of the carrier wave to the second frequency after the output unit outputs the change data to the audio signal processing circuit.