PURPOSE: To provide the method and device for correcting the output of a position detector in a modulator for antilock brake which correctly detects the position of a crank by correcting the gain and offset of a position detector.

CONSTITUTION: A brake control system 20 is equipped with a crank pin 44, DC motor 40 for deflecting the crank pin 44, potentiometer 50 for detecting the position of the crank pin 44, and a motor control circuit 74, and the motor control circuit 74 energizes the DC motor 40 at the engine start, and deflects the crank pin 44, and reads the values of the potentiometer 50 at the upper and lower limit positions, and calculates the correction coefficient of the potentiometer 50 on the basis of the above-described values. Then, the value of the potentiometer 50 which is read during brake control is corrected on the basis of the correction data, and the position of the crank pin 44 can be detected correctly.