The device has a differential plunger axially movable in a stepped bore formed in a housing for modulating a first fluid pressure supplied thereto when the pressure exceeds a critical magnitude, wherein one end face of the plunger is exposed exclusively to a second fluid pressure of which modulation is unintended, and the other end face is isolated from the first and second fluid pressures but receives such a compression force from a spring installed thereon between a piston that the critical magnitude is raised if the second fluid pressure is not supplied to the former end of the plunger. The piston is exposed to the first or second fluid pressure through a normally open valve, and a movable member is disposed in the housing, which member moves and closes the valve when the device is subjected to a deceleration larger than a predetermined magnitude, so that the critical magnitude of the first fluid pressure is raised as the weight of the vehicle increases.