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.