Rotational speed detection method using a photo-electric rotary encoder is widely used in robotic systems. When a small-sized rotary encoder with low number of pulse per revolution is used, error of the speed obtained from counting the encoder pulses is increased at lower speed of revolution. This paper proposes a rotational speed detection method of high resolution, which can be applied to a small-sized rotary encoder of 100 pulse per revolution. This method utilizes, at low speed range, analog signal of the encoder output pulse, the waveform of which is approximately triangle. By examining relation between the angle of rotation and the encoder two-phased output waveform, we propose an algorithm of detecting precisely the rotation angle from the two-phased output signals. The algorithm is implemented using a one-tip microprocessor. Rotational speed detected by the proposed method is compared with the accurate speed obtained from the potentiometer. Error of the proposed method is less than 6.8% at the low speed revolution of 1.8_??_80 rpm, which shows the usefulness of the method.