Variable Flux Permanent Magnet Synchronous Machine(VFPMSM) has been proposed for the purpose of high efficiency motor. However, in order to perform magnetization control of VFPMSM and optimum design, it is necessary to measure the operating point of the permanent magnet. Since the motor is located in the open magnetic circuit, the effect of the demagnetization field cannot be ignored. In general, the demagnetization coefficient is calculated from the permeance of the magnetic circuit, the demagnetization field correction is performed. However, there is a problem that the formula is complicated and errors between calculated value and experimental values are existed. Therefore, in this paper, we discuss a demagnetization field correction method using the slope of the hysteresis curve. The purpose of this study is to obtain the operating point of the permanent magnet under the motor driven condition by simple and accurate demagnetization field correction.