Sensorless control of brushless DC motors (BLDCMs) by detecting back electromotive force (back-EMF) is widely used because of ease of equipment. As conventional method, back-EMF detecting method by using phase lock loop (PLL) is used. This method uses the nature that back-EMF of γ-axis is related to estimated angular error. As usual, back-EMF of d-axis is zero in the case of using position sensor. To compensate estimated angular error becomes zero, PLL is used. However, this method assumes that estimated angular error is very small so that PLL is not able to deal with sudden disturbance. In this paper, back-EMF detection method by using back-EMF estimation observer (BEOB) is proposed. Proposed method is able to derive not only estimated angle error but also estimated angular velocity directly. To estimate angular velocity correctly, disturbance observer (DOB) is able to be applied, and robust angular velocity control can be achieved. Then, validation under change of back-EMF constant is conducted with experiments. Back-EMF constant changes due to parameter error and environmental temperature. Therefore, comparison of PLL, method and proposed method under change of back-EMF constant is conducted. Validity of proposed method was confirmed by experiments.