To reduce an operation force in an electric booster device, when a brake operation is performed while an electric motor becomes inoperable by fault of an electric system, etc.

A pressing member 140 is promoted through a ball screw mechanism 120 by controlling the operation of an electric motor 105 by a controller according to the stroke of a brake pedal, and the brake fluid pressure is generated by pushing a primary piston 178. A constant reaction force is provided to the stroke of the brake pedal by a reaction force spring 152. When the electric motor 105 is inoperable, a first detach mechanism 190 separates a plunger 104 from a movable spring receiver 151 so that the reaction force of a reaction force spring 152 may not act on the plunger, and a second detach mechanism 191 engages the plunger 104 to the direct acting member 121, and generates the brake fluid pressure by directly pushing the direct acting member by the plunger 104 by the rotational position of a direct acting member of a ball screw mechanism 120.