Applying an alternating current to a magnetic head as a sample generates an alternate-current magnetic field from the sample. A cantilever includes a probe that is made of a magnetic material or is coated with a magnetic material. The cantilever is displaced when it approaches the sample. Detecting the displacement of the cantilever detects distribution of the magnetic field from the sample. It is possible to fast measure distribution of the magnetic field generated from the sample when a frequency of the alternating current applied to the sample differs from a resonance frequency of the cantilever.