A brake force amplifier for a multiple-circuit brake system. The brake force amplifier has a multiple-circuit brake valve, parallel to which a switching valve is provided. This switching valve is actuated by a main cylinder pressure in exactly the same manner as the brake valve. When there is no pressure available in a presurizing apparatus supplying the brake force amplifier with pressure medium, the switching valve keeps the brake valve ineffective. When a brake force amplifier of this kind is used in an anti-skid apparatus, an apparatus is created which has lower cost, lower weight and a smaller structural volume than known designs and the function of which is improved in terms of noise and retroactive effect on the pedal. The brake force amplifier is intended for a multiple-circuit brake system for motor vehicles.