A controlling device used in a dual hydraulic braking system of a vehicle including a master cylinder which produces hydraulic braking pressure in response to the pedal depression, wheel cylinders disposed on each wheel, two mutually independent circuits for delivering the braking pressure from the master cylinder to each wheel cylinder, and a regulating valve disposed at least in one circuit of the two for controlling the hydraulic braking pressure delivered to the wheel cylinder. The controlling device is characterized in possession of an operation restricting mechanism, wherein a second piston, disposed substantially perpendicularly to a first piston in the regulating valve, is exposed to air on one end thereof, and affected by the hydraulic pressure on the other end thereof from a circuit which is not under control of the regulating valve, and is constantly biased by a compression spring toward the pressure receiving side thereof to block the reciprocal movement of the first piston, exceptionally allowing the first piston to reciprocate only when the hydraulic pressure acting on the second piston exceeds a certain predetermined value to overcome the spring force to shift the second piston toward the air-exposed end.