PURPOSE: To provide a pressure detector that is usable together with a pressure circuit for plural systems in common, and also capable of detecting the extent of pressure at the maximum pressure side.

CONSTITUTION: This is one that pertains to an electronic control brake system which controls hydraulic pressure in a pressure generator 11 according to that in a master cylinder 1, feeding a wheel cylinder 2 with this hydraulic pressure and generating an extent of braking farce. In a pressure detector 20 to be installed in two main circuits 3 and 4 of the master cylinder 1, two pistons 22 and 23 are installed in a bottomed tubular cylinder 21 free of sliding motion, and it is so structured that hydraulic pressure P1, P2 in the main circuits 3 and 4 of the master cylinder 1 are led to two pressure chambers 24 and 25 to be formed in space between the piston 22 on one side, a cylinder bottom part and the piston 23 on the other, through which the piston 23 at the opening side is shifted by dint of pressure at the maximum pressure circuit side, and a probe 13a of a load cell 13 is pressed at the end face, detecting the maximum pressure.