Regular maintenance of highway is an important issue to ensure safety of the vehicles using the road. Most of existing method of highway inspections are destructive, which take much times, efforts, and costs. In this paper, we propose GPR (Ground Penetrating Radar) imaging to detect possible defect of the road. GPR scanning on a plane parallel to the road yields 3D images, so that slice-by-slice images can be generated for a comprehensive evaluation. First, we simulate the subsurface-scanning with GPR-Max software, by setting up the parameters similar to expected real-condition. Then, we set up the experiment in our GPR Test-Range, in which a Network Analyzer is employed as a GPR. We compare and analyze both of the simulation and Test-Range results, including slice analysis, to asses the quality of the method. Our results indicates implementability of such 3D GPR imaging for road inspection.