An optical coherent tomography diagnostic apparatus (300, 900) comprising: a light source for outputting light (201, 409); a splitter for splitting the light outputted from the light source into a measuring light, a reference light and a standard light; a measuring light path connected to the splitter to transmit the measuring light; a reference light path connected to the splitter to transmit the reference light; a standard light path connected to the splitter to transmit the standard light; a probe insertable into a body cavity, the probe being connectable to the measuring light path to emit the measuring light to a subject of measurement, with light being reflected from the subject of measurement as reflection light; an image forming unit for calculating intensity distribution of the reflection light in an emission direction of the measuring light and for forming a tomographic image of the subject of measurement based on the intensity distribution by rendering the reflection light from the subject of measurement, which was obtained by the probe to have interference with the reference light; characterized in that a controllable shutter portion is provided in the standard light path, and a photodetector being provided at the output of the splitter for detecting - the coherent light interference between the standard light and the reference light in a check mode in which the shutter portion is opened and the probe is disconnected from the measurement light path, or - the coherent light interference between measurement light and the reference light in a measurement mode in which the shutter portion is closed and the probe is connected to the measurement light path, a demodulator unit for performing a demodulation process for extracting only a signal component of a coherent light signal outputted from the photodetector and for outputting said extracted signal component to an A/D converter for generating coherent light digital data by sampling said coherent light signal from the demodulator unit and for inputting the coherent light digital data into a signal processing unit, said signal processing unit being capable of calculating a time change of the coherent light digital data inputted from the A/D converter in the check-mode and being caused by change of the path length of the reference light path in a high-speed manner corresponding to an inspection range in a depth direction of the subject to be measured. a light source for outputting light (201, 409); a splitter for splitting the light outputted from the light source into a measuring light, a reference light and a standard light; a measuring light path connected to the splitter to transmit the measuring light; a reference light path connected to the splitter to transmit the reference light; a standard light path connected to the splitter to transmit the standard light; a probe insertable into a body cavity, the probe being connectable to the measuring light path to emit the measuring light to a subject of measurement, with light being reflected from the subject of measurement as reflection light; an image forming unit for calculating intensity distribution of the reflection light in an emission direction of the measuring light and for forming a tomographic image of the subject of measurement based on the intensity distribution by rendering the reflection light from the subject of measurement, which was obtained by the probe to have interference with the reference light; characterized in that a controllable shutter portion is provided in the standard light path, and a photodetector being provided at the output of the splitter for detecting - the coherent light interference between the standard light and the reference light in a check mode in which the shutter portion is opened and the probe is disconnected from the measurement light path, or - the coherent light interference between measurement light and the reference light in a measurement mode in which the shutter portion is closed and the probe is connected to the measurement light path, - the coherent light interference between the standard light and the reference light in a check mode in which the shutter portion is opened and the probe is disconnected from the measurement light path, or - the coherent light interference between measurement light and the reference light in a measurement mode in which the shutter portion is closed and the probe is connected to the measurement light path, a demodulator unit for performing a demodulation process for extracting only a signal component of a coherent light signal outputted from the photodetector and for outputting said extracted signal component to an A/D converter for generating coherent light digital data by sampling said coherent light signal from the demodulator unit and for inputting the coherent light digital data into a signal processing unit, said signal processing unit being capable of calculating a time change of the coherent light digital data inputted from the A/D converter in the check-mode and being caused by change of the path length of the reference light path in a high-speed manner corresponding to an inspection range in a depth direction of the subject to be measured.