A magnetic field observation device that observes a stray magnetic field from a magnetic material specimen, the magnetic field observation device comprising: a probe that has a magnetic moment which more easily undergoes magnetization reversal than the magnetic material specimen; an excitation mechanism that excites the probe; a scanning mechanism that relatively moves the probe and the magnetic material specimen to make the probe scan a surface of the magnetic material specimen; an alternating magnetic field generation mechanism that can make the probe periodically undergo magnetization reversal and that applies, to the probe, an alternating magnetic field having such a magnitude as not to make the magnetic material specimen undergo magnetization reversal; and a modulation measurement mechanism in which a degree of periodical frequency modulation of oscillation of the probe generated by change of an apparent spring constant caused by an alternating force applied to the probe through a magnetic interaction between magnetization of the probe and magnetization of the magnetic material specimen, wherein a direction of magnetization is periodically changed by the alternating magnetic field applied by the alternating magnetic field generation mechanism, can be measured by frequency demodulation or by measuring an intensity of one sideband wave spectrum among sideband wave spectrums generated by the frequency modulation; characterized in that a product of an intensity of a residual magnetic pole of a tip of the probe that is not changed even by application of the alternating magnetic field from the alternating magnetic field generation mechanism and a gradient of the alternating magnetic field applied from the alternating magnetic field generation mechanism to the probe is smaller than a product of an intensity of a magnetic pole of the tip of the probe that is changed by the application of the alternating magnetic field from the alternating magnetic field generation mechanism and a gradient of a magnetic field applied from the magnetic material specimen to the probe. a probe that has a magnetic moment which more easily undergoes magnetization reversal than the magnetic material specimen; an excitation mechanism that excites the probe; a scanning mechanism that relatively moves the probe and the magnetic material specimen to make the probe scan a surface of the magnetic material specimen; an alternating magnetic field generation mechanism that can make the probe periodically undergo magnetization reversal and that applies, to the probe, an alternating magnetic field having such a magnitude as not to make the magnetic material specimen undergo magnetization reversal; and a modulation measurement mechanism in which a degree of periodical frequency modulation of oscillation of the probe generated by change of an apparent spring constant caused by an alternating force applied to the probe through a magnetic interaction between magnetization of the probe and magnetization of the magnetic material specimen, wherein a direction of magnetization is periodically changed by the alternating magnetic field applied by the alternating magnetic field generation mechanism, can be measured by frequency demodulation or by measuring an intensity of one sideband wave spectrum among sideband wave spectrums generated by the frequency modulation; characterized in that a product of an intensity of a residual magnetic pole of a tip of the probe that is not changed even by application of the alternating magnetic field from the alternating magnetic field generation mechanism and a gradient of the alternating magnetic field applied from the alternating magnetic field generation mechanism to the probe is smaller than a product of an intensity of a magnetic pole of the tip of the probe that is changed by the application of the alternating magnetic field from the alternating magnetic field generation mechanism and a gradient of a magnetic field applied from the magnetic material specimen to the probe.