Humanoid robots have discrepancies between the postures simulated on models and those of actual humanoid bodies. To correct this problem, this paper proposes a method in which a robot observes its own posture in a mirror. This method does not need an additional time-consuming setup, such as calibration of external cameras. We attach a reference point to a known position on the robot body. By observing markers and the reference point in the same view, we can estimate the position of the markers accurately based on the geometric relationship between the mirror, an object, and its mirror reflection. We evaluate the proposed method by experiments of measuring and correcting the posture of an actual robot body to attain the target posture.