Disclosed herein is a method for quantifying a pigmented lesion using Optical Coherence Tomography (OCT). The method includes (a) irradiating light and receiving an interference signal produced by reflection of the light from first and second boundary layers of a pigmented lesion; and (b) calculating size information of the pigmented lesion using phase information of the interference signal. According to embodiments of the present invention, there is an advantage of allowing calculation of size information of a pigmented lesion using OCT, by increasing a measurement range in the axial direction to which beams are irradiated.