An electrically actuated booster performs displacement control such that the relative displacement relationship between an input member and an assist member is variable according to the absolute displacement of the input member. A target displacement that makes variable the relative displacement relationship between an input piston and a booster piston is set according to a detection signal from a potentiometer (86), and displacement control is performed so that the relative displacement between the two pistons becomes equal to the target displacement on the basis of a signal from a relative displacement sensor (100) that detects the relative displacement between the two pistons.