1. A brain current source estimating method for estimating, based on an electromagnetic field observed outside a scalp, a position of a current source as a source of said electromagnetic wave existing in the brain, comprising the steps of setting, in the brain, a plurality of virtual curved surfaces having depths from brain surface different from each other and shapes not intersecting with each other, and setting lattice points on each of said virtual curved surfaces; estimating, on each of said virtual curved surfaces, a: current distribution for recovering said observed electromagnetic field; and based on an expansion of the current distribution estimated on said virtual curved surfaces and a difference between the electromagnetic field recovered based on said current distribution and said observed electromagnetic field, identifying a virtual curved surface at which said expansion and said difference attain relative minimums among said plurality of virtual curved surfaces as a true curved surface on which said current source exists.