In the GAMMA 10 tandem mirror, the low frequency fluctuations are studied in relation to plasma parameters. By using optical detectors in the central cell, the flute-like instabilities with m = −1 are analyzed. The plasma column rotates as a rigid body in the direction of the E × B drift. It has been observed that instabilities can be suppressed by inserting conducting plates into the plasma periphery at the outer transition region between the anchor and the plug/barrier cells. The plasma stabilization mechanism of the conducting plates is studied.