Light emitted from a light source is divided into measuring light and reference light. The reflected light from the object and the reference light are superposed. Interference light of the reflected light and reference light which have been superposed is detected. Intensities of the reflected light in a plurality of positions in the direction of depth of the object are detected on the basis of the frequency and the intensity, and a tomographic image of the object is obtained on the basis of the intensity of the reflected light in each position in the direction of depth. A compensating signal is obtained by removing the spectral components of the measuring light from an interference signal obtained by detection of the interference light, and the compensating signal is provided for detection of the intensities of the reflected light after a Gaussian transform.