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The high-frequency carrier type magnetic field sensor, it is also called the GMI sensor, is one of the high resolution magnetic field sensor. The high-frequency carrier type magnetic field sensor is based on the principle that the impedance of a magnetic thin film is changed when a magnetic field is applied. Changing the impedance of the sensor also changes the phase characteristics of the sensor. We directed our attention to the change of the phase characteristic and measured the magnetic field by measuring the phase. In general, the phase detection method is less effect on an external noise, it is expected to a high signal to noise ratio. However, the high-frequency carrier type magnetic field sensor is optimized to the impedance detection, it is not the appropriate sensor to measure the magnetic field by detecting the phase difference. To highly improve the sensitivity, we suggest the use of the new magnetic field sensor for phase detection. We investigated that our phase detection type magnetic field sensor has highly improved the sensitivity by controlling an annealing temperature.