Abstract: A method of manufacturing a magnetic sensor element, and a security system using this element. The method involves preparing an amorphous magnetic alloy having an atomic composition ratio of ?Co.sub.x Fe.sub.1-x !.sub.1-n ?B.sub.y Si.sub.1-y !.sub.n where x=0.about.1, y=0.2.about.1, n=0.1.about.0.3, and heat treating the magnetic alloy in the atmosphere for more than one minute at 200.degree. C..about.460.degree. C. to produce the magnetic sensor element where the heat treatment is carried out under substantially zero external magnetic field. The manufacturing method results in a magnetic sensor element which produces a sensor signal having two major pulses and two minor pulses during one period of an externally applied alternating magnetic field, the two major pulses being easily detected by a security system.