power source is cut therefore even if the robot is moved for four days or more due to a production line change in a factory the absolute value is held so that the robot can be immediately restarted led are used as the light emitting device however the conventional methods described in fig1 and 11 have problems therefore in the case of 15 bits as shown in fig1 three leds 4 are used the light emitting positions are grouped and the grouped leds emit light on the other hand in the case of grouping if the light emitted from the central and peripheral light emitting positions are not parallel an accurate detection signal cannot be obtained therefore an aspheric lens is used in the lens section to thereby obtain parallel light since it is sufficient to use parallel light and to emit light from each group a small lens can be used in the led thus since the light emitting source has a short focal distance the encoder is shortened fig5 is a diagram showing a pattern of the absolute encoder fig6 is a circuit diagram of a portion of the absolute encoder in fig1 to eliminate the drawback due to variations in light emitting devices and photo sensing devices shown in fig1 and 12 the pattern of the absolute encoder is divided into three groups as described above in the groups transparent reference tracks d 1 d 2 and d 3 are provided as shown in fig5 a one chip silicon photodiode array is used as a photo sensing devices 6 in fig5 circles s a s b and s c indicated by broken lines show areas where light.