An optical coherent tomographic image forming apparatus including a light guide configured to divide a light into a measurement light and a reference light； a flush unit configured to discharge a flush liquid into a body lumen； a refractive index setting unit configured to set a refractive index of the flush liquid； an adjustment value setting unit configured to set an adjustment value； a calibration unit for adjusting the length of the reference optical path or the length of the measurement optical path based on the adjustment value； an image construction unit configured to construct a cross-sectional image； and an offset value calculation unit for calculating an offset value of the length of the reference optical path based on a changed refractive index when the refractive index of the flush liquid is changed by the refractive index setting unit after calibration is executed by the calibration unit.
