In a magnetic field source detecting apparatus, a magnetic sensor unit detects an intensity and a direction of a measurement target magnetic field on or over a surface of a test target object; and a position estimating unit estimates a position in a depth direction of a magnetic field source that exists at an unspecified position inside a test target object on the basis of the intensities and the directions of the measurement target magnetic field detected by the magnetic sensor at at least two 2-dimensional positions of the surface.