Experiments using a single feedback loop, in which electrostatic signals from a plasma were amplified and fed back as potentials to the plasma boundary, have been carried out in the PHOENIX-II mirror machine working as a simple mirror. The effect of the feedback system on plasma losses caused by the flute instability has been studied and a correlation between plasma losses and the amplitude and frequency of the instability has been obtained. It is concluded that plasma losses can be controlled, to a certain extent, by a simple feedback system.