1. A method comprising: increasing the number of elements of a wavelength spectrum of interfering light by using an analysis unit, wherein said interfering light is caused by interference of return light from an eye irradiated by measurement light via a scanning unit and reference light corresponding to said measurement light, wherein the scanning unit scans the measurement light on a retina of the eye with a cornea acting as a fulcrum; converting the wavelength spectrum into a wavenumber spectrum by using an analysis unit after the number of elements of the wavelength spectrum is increased; and acquiring a tomographic image of the retina based on said wavenumber spectrum by using an analysis unit.