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In the construction of a highly sensitive magnetic sensor, the thick superconducting high-critical temperature superconductor (HTS) film will generally break the superconducting state of current flow at a value of current density Jms greater than that of the critical current density Jc , when the resistance Rms of the film to the flow of current occurs in the absence of an excitation magnetic field. The magnetic sensitivity S of the sensor when exposed to an excitation magnetic field, was found to be about 200%/(10-4 T) for Bi-Pb-Sr-Ca-Cu-O (BPSCCO) magnetic sensor, and about 7%(10-4 T) for Y-Ba-Cu-O (YBCO) magnetic sensor. That is, the magnetic sensitivity of the BPSCCO magnetic sensor is about 200 times greater than that of a giant magnetoresistance (GMR) sensor. Furthermore, after measurements through more than 300 thermal cycles between temperatures of 77.4 K and 300 K, the characteristics of the thick BPSCCO film magnetic sensor were found to exhibit no significant changes in magnetic response. The present paper reveals the dependencies of Rms on the temperature T, of resistivity ρ of the thick film on Jms, of Rms on the applied excitation magnetic field Bex, and the dependence of S on the ρ.