A magnetic-field-observation device and method for measuring magnetic force near a magnetic material specimen's surface with high resolution and detecting the polarity of the magnetic pole of specimen's surface. The device including: a probe; excitation mechanism that excites it; scanning mechanism that relatively moves the probe and specimen; alternating magnetic field generation mechanism to make the probe periodically undergo magnetization reversal and apply thereto an alternating magnetic field having magnitude not making the specimen undergo magnetization reversal; and modulation measurement mechanism for measuring degree of periodical frequency modulation of the probe's oscillation caused by its apparent spring constant periodically changed by force of periodically changed intensity and applied to the probe by alternating force through magnetic interaction between magnetizations of the probe and specimen, by frequency demodulation or by measuring intensity of one sideband wave spectrum among spectrums generated by the frequency modulation. The method performed using the device.