This invention relates to a brake force booster for automotive vehicles, having a panic braking function which can increase output during an emergency braking situation. The brake force booster consists of a front and a rear housing which are bonded together to create an internal pressure chamber; a diaphragm which compartmentalises said pressure chamber; a power piston forming a penetration hole, through said diaphragm and installed into said rear housing; a valve seat which contains a longitudinally formed penetration hole and is installed to move forward/backward and give support to said power piston; a seal which seals between the inner circumferential surface of said power piston and the outer circumferential surface of said valve seat; a push rod which receives the operational power of the brake pedal and is elastically supported rearward of said power piston; a control valve which is installed to move forward/backward in said penetration hole within said power piston and is supported elastically toward the front of said push rod; an air valve which is installed in said penetration hole of said valve seat and moves forward/backward in said power piston and which is supported elastically toward the front of said push rod so that it can be operated to control the air flow toward said diaphragm according to the contact of said control valve; and a stopper which is attached to said power piston and limits the rearward movement of said valve seat. Said air valve has a protrusion protruded radially outward and the end of said valve seat is coupled to said stopper when said air valve is closed. The first leg of multiple stratiform legs which form the control, touches said protrusion. The first leg of said multiple is formed in such a way that that said end can be selectively separated from said stopper, since the leg can be selectively separated radially outward by the protrusion of said air valve, which depends on the forward movement speed of said air valve. When the end of said first leg is separated from said stopper, said valve seat is provided with a reaction control mechanism causing it to stop after moving a pre-determined distance against said stopper.