An optical coherent tomographic image forming apparatus comprising: a light source; a light guide (226, 230) configured to guide a light outputted from the light source and divide the light into a measurement light and a reference light, the light guide including a measurement optical path and a reference optical path; an optical probe having a distal end and a proximal end, the distal end being configured for insertion into a body lumen; a flush unit (115, 116, 118) including a flush liquid container and a flush liquid discharger, the flush unit (115, 116, 118) configured to discharge a flush liquid into the body lumen; a refractive index setting unit configured to set a refractive index of the flush liquid; an adjustment value setting unit (113, 112) configured to set an adjustment value for adjusting a difference between a length of the measurement optical path and a length of the reference optical path, the adjustment value being determined using the refractive index of the flush liquid; a calibration unit comprising an optical path length adjuster 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 based on a light intensity of an interference light that is obtained from a reflected light obtained by emanating the measurement light from the optical probe toward a biological tissue defining the body lumen while the flush liquid subsists in the body lumen; 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. a light source; a light guide (226, 230) configured to guide a light outputted from the light source and divide the light into a measurement light and a reference light, the light guide including a measurement optical path and a reference optical path; an optical probe having a distal end and a proximal end, the distal end being configured for insertion into a body lumen; a flush unit (115, 116, 118) including a flush liquid container and a flush liquid discharger, the flush unit (115, 116, 118) configured to discharge a flush liquid into the body lumen; a refractive index setting unit configured to set a refractive index of the flush liquid; an adjustment value setting unit (113, 112) configured to set an adjustment value for adjusting a difference between a length of the measurement optical path and a length of the reference optical path, the adjustment value being determined using the refractive index of the flush liquid; a calibration unit comprising an optical path length adjuster 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 based on a light intensity of an interference light that is obtained from a reflected light obtained by emanating the measurement light from the optical probe toward a biological tissue defining the body lumen while the flush liquid subsists in the body lumen; 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.