Field effect transistors (FETs) whose channel is composed of a network of single wall carbon nanotubes (SWNTs) have been studied to investigate the mechanism of the device operation via scanning gate microscopy (SGM) at room temperature. We observed different SGM response in networks of SWNTs either synthesized by CoMoCAT process or semiconducting enriched by density gradient ultracentrifuge process. In the former case, SGM response was observed at specific inter-tube junctions suggesting a Schottky junction formed with semiconducting and metallic SWNTs in the network. In contrast, multiple concentric rings in the SGM response are observed within the tubes in a network of the latter SWNTs suggesting a possibility of quantum mechanical transport at room-temperature. Different type of SGM responses are confirmed in the two kinds of SWNTs networks, nevertheless such active positions would likely have an important role in the FET operation mechanism in each network.