The writer proposed a method for finding the position, intensity and direction of a underground magnetic dipole from the corresponding magnetic anomaly on the earth's surface. In the first place, the distribution of the vertical force is expressed as follows;With the aid of some relations concerning spherical harmonic functions, then, the coefficients of the spherical harmonic expansion of the magnetic potential on a sphere that entirely contains the magnetic matter are determined. The radius of the sphere which is included in the expression as a parameter adjusted so as to make the higher degree terms minimum in the next place.Applying the method to the magnetic changes which accompanied the eruption of Miyakezima Island of 1940, a reasonable conclusion is obtained.