The present invention relates to an electrically driven brake booster in which boost control is performed as follows; when an input piston is displaced forward by an input rod upon an operation of a brake pedal, an electric motor is operated according to the input, and a primary piston is displaced forward by a ball-screw mechanism, and then a primary and secondary pressure chamber in a master cylinder main body are pressurized. A first seal and a second seal are disposed at a seal portion between the primary piston and the input piston. A portion between the first seal and the second seal is in communication with an outside through a discharge passage. Brake fluid which leaks from the first seal is sealed off by the second seal, and is discharged to the outside through the discharge passage, whereby it becomes possible to prevent that the electric motor and the ball-screw mechanism otherwise might be corroded by the leaking brake fluid.