Abstract: It is experimentally proved that the butane can be used as an operating gas of the pro portional counter. The operating performances of the counter with the butane are quite similar to that with the methane. A KX 50 butane flow proportional counter is described and its plateau characteristic is determined. For this counter, the plateau length, the slope of the plateau, the background count rate is 600 V, 1.7 % per 100 V, 0.02 s -1 for 239 Pu α source and about 500 V, 1.8 % per 100 V, 3 s -1 for 90 Sr 90 Y source, respectively.