PROBLEM TO BE SOLVED: To suppress the appearance of radial noise by a stent or the like while detecting a weak biosignal with high sensitivity in the formation of a tomographic image by wavelength sweep type optical interference.SOLUTION: In the optical interference tomographic image forming apparatus wherein the light output from a wavelength sweep light source is divided into measuring light and reference light to get the measuring light emitted into the body cavity while rotating an irradiation direction through the probe inserted in the body and the tomographic image in the body cavity is formed on the basis of the intensity of the interference light obtained from the reflected light from the body cavity and the reference light, the intensity of the interference light is converted to an electric signal to turn the converted output signal logarithmically amplified, and the logarithmically amplified signal is discretely sampled to be converted to digital interference data. The digital interference data is divided at every irradiation direction to be converted to line interference data and inverse logarithmic conversion is applied in the magnification corresponding to the intensity of the signal at every line interference data to demodulate the line interference data, so that the demodulated line interference data is converted to the data of a frequency domain to construct the tomographic image.
