The ultra-sensitive susceptibility detection apparatus of anharmonic resonance measurement type using an atomic magnetometer detects a change in susceptibility by a specimen containing an object to be measured. The apparatus includes an atomic magnetometer. The atomic magnetometer includes a cell containing an alkaline metallic atom, a light source for magnetically polarizing the alkaline metallic atom of the cell, and a bias magnetic field applicator for applying a bias magnetic field to adjust a measuring resonance frequency of the alkaline metallic atom. The apparatus includes an excitation magnetic field applicator for applying an excitation magnetic fields of different frequencies to magnetically excite the specimen, but not to couple the excitation field directly to the measuring atomic resonance frequency, and a measuring device for measuring a change in magnetic polarization of the alkaline metallic atom, which is affected by a magnetic field caused by the specimen being magnetically excited by the excitation magnetic field.