PURPOSE: To improve slip prevention performance response in a brake fluid pressure control device having the capability of preventing the locking of a wheel under braking and excessive slip under acceleration, and equipped with a brake fluid pressure adjustment mechanism between a master cylinder and a wheel cylinder.

CONSTITUTION: Both pressure changing chambers 24a and 24b of a brake booster 20d connected to a master cylinder 20a are made continuous to each other via a communication passage 27, and this passage 27 is fitted with an electromagnetic selector valve 10d. The operation of this valve 10d is selected, upon occurrence of excessive slip under acceleration, thereby generating differential pressure between the negative pressure chamber and pressure changing chamber of the booster 20d for the operation thereof.