An optical coherent tomography diagnostic apparatus including: a light source (409)； a splitter (408) for splitting the light outputted from the light source into a measuring light and a reference light； a measuring light path； a reference light path； a probe (401) inserted into a body cavity and emitting the measuring light to a subject of measurement； an image forming unit (410) for calculating intensity distribution of the reflection light and for forming a tomographic image； a standard light path (433) for transmitting standard light obtained by further splitting the light； wherein the light path length when exerting interference between the standard light and the reference light is approximately equal to the light path length when exerting interference between the reference light and the reflection light, and there is included a calculation unit for calculating time change of coherent light data obtained by exerting interference between the standard light and the reference light.
