A deceleration sensing type fluid pressure control device for vehicle brake system having a control piston; a proportioning action mechanism capable of varying the reducing value of output pressure from input pressure in relation to a load imposed on the control piston; an adjusting piston one end of which is connected to a fluid pressure source through a passage arrangement; and a valve mechanism which is disposed in the passage arrangement and closes when the deceleration of the vehicle reaches a predetermined value. The fluid pressure control device is provided with a swaying lever which has a middle point thereof serving as a fulcrum. One end of the lever engages the control piston. Between the other end of the lever and the other end of the adjusting piston, there is provided a control spring which imposes the above stated load on the control piston.