To provide a concealed matter searching method capable of certainly searching concealed matter having a small diameter.

In the concealed matter searching method wherein electromagnetic waves are emitted to a medium 20 from an antenna while allowing the antenna to scan on the surface 20a of the medium 20 concealing objects 1 and 2 and a reflection signal is received by the antenna to form three-dimensional voxel data having the position (x, y) on the surface 20a of the medium 20 and a reflection time (t) as coordinates (x, y, t) to search the positions of the objects 1 and 2, a process for performing scanning along a plurality of scanning lines 4a parallel to a first direction to form and store first voxel data, a process for performing scanning along a plurality of scanning lines 4b parallel to the second direction crossing the first direction to form and store second voxel data and a process for calculating third voxel data on the basis of the addition processing result of the first and second voxel data at every coordinates (x, y, t) are performed.