An absolute position length measurement type encoder includes: a scale which has an ABS pattern based on a pseudorandom code and forms bright and dark patterns by the ABS pattern; a light-receiving element for receiving the bright and dark patterns; a binarization circuit for dividing a digital signal in accordance with a signal output from the light-receiving element into a mode having high brightness and a mode having lower brightness, carrying out a binarization process by a threshold value where dispersion with regard to the two modes is maximized, and decoding the pseudorandom code by the unit of minimum line width of the ABS pattern from the binarized values; and a position detection circuit for calculating the absolute position of the scale with respect to the light-receiving element by acquiring the correlation between the decoded pseudorandom code and the design value of the pseudorandom code. Thereby, binarization errors of signals produced by the ABS pattern can be reduced, and highly accurate and stabilized measurement of an absolute position is carried out.