A study of fluctuation is one of main issues in fusion plasma researches. In the tandem mirror GAMMA10, a potential fluctuation changes during the formation of confinement potential with the application of electron cyclotron heating (ECH). Potential fluctuation induces radial particle transports. We have studied the radial profiles of the potential fluctuation during the application of ECH. The potential in the central cell of GAMMA10 is measured by a gold neutral beam probe (GNBP) system. GNBP is a useful tool for the study of radial potential and its fluctuation directly in the fusion plasmas. It was confirmed that the stronger radial potential was formed during the application of plug ECH than that without plug-ECH.