A gas discharge switch for controlling low level 3 cm-band signals was developed by modifying a iB24 TR tube. The keep-alive electrode in the tube was placed nearer to the r-f gap, and the aperture of the lower cone was made larger than that in the standard tube. It was observed that a keep-alive depth (distance from r-f gap) of 0.4 mm gave the highest attenuation. Among the various gas fillings tried, nitrogen was found to be most suitable. It gave a high attenuation and a reasonably long tube life. About 44 db attenuation was obtained at a keep-alive current of 0.4 ma with a gas rilling of 12 mm Hg of nitrogen. No detectable r-f noisc was introduced by the keep-alive discharge. A qualitative discussion is presented to account for the attenuation in terms of the motion of the electron furnished by the keep-alive discharge under an r-f field.