1. An object tracking system using a robot, comprising: an image acquisition unit that acquires images in front of the robot; an object detector that detects an object from the images acquired by the image acquisition unit; a motion pattern calculator that calculates a motion pattern including a distance from the robot and a motion direction of the robot with respect to the object detected by the object detector; a motion pattern storage unit that stores the motion pattern calculated by the motion pattern calculator; and a current position calculator that calculates a current position of the object based on the motion pattern for the object stored in the motion pattern storage unit when the object disappears from the images currently acquired by the image acquisition unit, wherein tracking for the object is performed by moving the image acquisition unit based on the current position calculated by the current position calculator; wherein the object detector includes: a body region detector that detects a body region for the detected object; a tracking region setting unit that sets a tracking region for the detected object based on the body region detected by the body region detector; and a leg pattern detector that detects a leg pattern within the tracking region set by the tracking region setting unit; and wherein the motion pattern calculator calculates the motion pattern of the object based on the leg pattern detected by the leg pattern detector.