A calibration jig allowing simple and repeatable calibration of a probe optical tomographic apparatus is disclosed. The jig includes a holding member removably attachable to an attachment section of the apparatus and a reflective surface held by the holding member. The reflective surface reflects measurement light emitted from an emitting section of the attachment section and directs reflected light back to the emitting section. If a probe of the apparatus is covered with a sheath, the jig may include a light transmitting member, which generates the same dispersion as dispersion at the sheath, between the emitting section and the reflective surface. The reflective surface may be a single reflective surface disposed within an area corresponding to twice a coherence length of the laser light with a zero path position of the reflective surface being the center of the area.