In most of the cases for terahertz near field microscopy, a free space propagating electromagnetic wave interacts with a small tip placed above the sample. In our original approach, we use a guided way to bring the terahertz wave and to improve the interaction efficiency with the sample. The idea is to take benefit of the confinement around a Planar Goubau Line (PGL). We have fabricated a planar system with an integrated Uni-Travelling-Carrier Photodiode (UTC-PD) as a broadband source. The system can be used to map continuously at discrete frequencies the sample in a wide frequency range.