Suppose gravity anomaly on a double-layer structure consisting of surface layer and underlying bedrock. We assume density contrast between them and mean depth to their boundary, and then we can calculate subsurface topography of the boundary by using, for example, Fourier series method or Fourier integral method. In these methods, elevation of the subsurface topography from the mean depth is assumed to be so small, that the second and higher powers of the elevation can be usually neglected. In some actual cases, such a neglect can not be correct. We propose in this paper a simple method for obtaining the subsurface topography without neglecting the second and higher powers of the elevation. The actual calculation result is shown in a figure.