A vacuum degree augmentation device for use in brake apparatus is provided in which a diaphragm-piston divides the body of a valve mechanism into two air-tight chambers, the valve mechanism is shiftable in response to the operation of a plunger which in turn operates in response to the operation of an input rod, the operation of the valve normally maintains the chambers in communication with each other under low pressure, but, during braking, one chamber is placed in communication with the open air to produce a differential pressure between the chambers so as to operate the piston, with the force operating the piston being transferred through a reaction lever, a fulcrum plate and an output rod to a master cylinder.