The scanning speed of a sample in Doppler OCT, etc., is increased to enable quick measurement of blood flow rate, blood flow volume, etc. Wideband light from a light source 2 is linearly polarized by a polarization controller 3 and this linearly polarized beam is split into vertically polarized light and horizontally polarized light using a Wollaston prism 14 at a sample arm, which are then irradiated simultaneously onto two different locations of the sample in the scanning direction using a galvanometer mirror 18, and reference light from a reference arm 5 and object light from a sample arm 6 are merged and caused to interfere with each other, with the resulting interference signal light passed through a diffraction grating 26 for spectroscopy, while the horizontal component and vertical component are separated by a polarized beam splitter 28, to simultaneously measure the components with two polarization-sensitive optical detectors 29, 30 and thereby obtain two tomography images of the same location at different times by one mechanical scan, thus allowing the amount of temporal change in phase to be measured using the two tomography images.