Interior Permanent Magnet Synchronous Motors (IPMSM) are an important component for a robotic application because of its high torque property. In order to drive IPMSM with high efficiency, feedback control with rotor position sensors is utilized. However, since those sensors make the instrument size large and the cost high, many researches on sensorless control which is feedback control without position sensors have been conducted. Especially for low-speed range, sensorless control with the use of additional high-frequency signals to detect rotor position has been proposed although these signals might cause rotor vibration or low efficiency. This paper proposes a novel position estimation method based on disturbance observer which detect position estimation error. The estimation method achieves detection of rotor position at low speed since rotor saliency and current derivative are utilized. This method differs from conventional observer-based methods in that the disturbance observer not only estimates the phase, but also estimates the term proportional to the axial error and applies a compensation voltage for it. This method also avoids to use any additional signals such as high-frequency voltage for the estimation. The effectiveness of the proposal was confirmed by simulations.