An optical tomographic diagnostic apparatus is characterized by executing a first step (S1) to acquire a wavelength spectrum, a second step (S2) to increase the number of elements of the wavelength spectrum, a third step (S3 and S4) to convert the wavelength spectrum into a wavenumber spectrum and to decrease the number of elements to provide a wavenumber spectrum of equal intervals, and a fourth step (S5) to acquire tomographic information of the object to be inspected from the wavenumber spectrum. As a result, a wavenumber spectrum of equal intervals can be obtained which is faithful to a physical phenomenon, and more accurate tomographic information can be obtained.
