Provided is an electric booster which can be downsized by enhancing space efficiency while reducing the effect of a magnetic field from outside on a magnetic sensor by means of a magnetic shield. The stroke amount of an input plunger 29 connected to a brake pedal 6 is detected by a stroke detection sensor. The operation of an electric motor is controlled by controller on the basis of the stroke amount of the input plunger to thrust a primary piston through a ball screw mechanism, and brake fluid pressure is thus generated by a master cylinder 4. The stroke detection device uses a Hall IC to detect magnetic flux densities from first and second magnet members mounted on the input plunger to obtain the stroke of the input plunger. The first and second magnet members and the Hall IC are disposed inside a linear-motion member formed of a cylindrical magnetic body of the ball screw mechanism to be magnetically shielded.