We have successfully characterized the dynamical fluctuation of electrical potential due to random telegraph noise (RTN) using MOSFETs with extra terminals for potential sensing. Among some cases of potential changes, devices with clear response in the extra terminals were analyzed in detail. It was found RTN can cause the potential fluctuation in the entire channel region. The magnitude of the fluctuation was consistent with those due to Vg in static properties. These results demonstrate the direct observation of channel potential changes due to number fluctuation phenomena.