An electrically actuated brake booster capable of obtaining a desired boost ratio for a given input pressure when an electric actuator is activated as a boost source, so as to ensure a desired operability of a brake pedal. The electrically actuated brake booster comprises: an input piston 22 to which an input thrust is applied through an input rod 9 that moves in coordination with operation of a brake pedal 8; and a booster piston 21 to which a booster thrust is applied by an electric actuator 40 that uses an electric motor 41 as a drive source, such that the input piston 22 and the booster piston 21 are disposed to be movable relative to each other but are normally maintained in a neutral position of relative displacement by means of springs 34. While an amount of relative displacement between the pistons 21 and 22 is determined by the potentiometer 45, the electric motor 41 is controlled to adjust the amount of relative displacement to a predetermined value. At the same time, a reaction force acting on the input piston 22 resulting from a hydraulic brake pressure is offset by urging forces of the springs 34 to thereby obtain a desired level of boost ratio.
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