A differential orthogonal modulation method and a differential orthogonal modulation apparatus using a repetition time interval difference of a chirp signal are disclosed. Such a differential orthogonal modulation method performed in a transmitter unit of a digital communication system comprises the steps of converting data having a serial bit format to be transmitted in the transmitter unit into a parallel format to create parallel input data, inputting the parallel input data into a symbol mapper, orthogonally modulating the parallel input data inputted into the symbol mapper to create modulated data comprised of differential bi-orthogonal functions, differentially encoding the modulated data to output encoded data, converting the encoded data into a serial bit format to create a baseband signal, and processing the baseband signal to create a transmission signal in a form of a chirp signal having a specific repetition time interval difference.