1. A two-beam optical coherence tomography apparatus, comprising: a wideband light source; a polarization controller which linearly polarizes the beam from the wideband light source; a sample arm which comprises: a Wollaston prism which splits the linearly polarized light into a vertically polarized beam and a horizontally polarized beam, or a clockwise circularly polarized beam and a counterclockwise circularly polarized beam by placing a polarization conversion element after the Wollaston prism; and a galvanometer mirror which performs scanning by simultaneously irradiating the vertically polarized beam and the horizontally polarized beam, or the clockwise circularly polarized beam and the counterclockwise circularly polarized beam, onto two different locations of a target sample in the scanning direction of the galvanometer mirror; a reference arm; a fiber coupler which merges reference light from the reference arm and object light from the sample arm and causes them to interfere with each other to produce interference signal light; and a spectrometer comprising: a diffraction grating which converts the interference signal light into spectral interference signal light by means of spectroscopy; a polarized beam splitter which separates the spectral interference signal light into a vertically polarized component and a horizontally polarized component; and two optical detectors which simultaneously detect the vertically polarized component and the horizontally polarized component of the spectral interference signal light to obtain two tomography images of the same location at different times by one mechanical scan as mentioned above, and the obtained two tomography images are used to measure the amount of temporal change in phase at the same location.