In order to reduce CO2 emissions, the European Union commenced an experimental approach called the "NaturalHy Project" in May 2004 to transport a mixture of hydrogen and the existing high-pressure natural gas in pipelines. NaturalHy denotes a mixture of hydrogen and natural gas. The properties of hydrogen-methane mixed gas should be investigated in detail because this gas is expected to replace methane as an energy carrier in the near future. Thus, the results of the authors would be useful to the researchers involved in developing pumps, compressors, or mass flow controllers for the next generation energy systems. To the best of their knowledge, this article is the first report on the viscosity measurement of hydrogen-methane mixed gas by using a quartz friction gauge. The authors measured viscosity using two methods for the purpose of comparison. In the first method, the pressure drop was measured in a laminar pipe flow. The authors prepared an electrically polished ultraclean smooth tube and carefully measured the pressure drop between upstream and downstream using a capacitance manometer. In the second method, the drag acting on the quartz friction gauge in a gas atmosphere was measured. The quartz friction gauge is sensitive to pressure, temperature, and viscosity. To achieve suitable conditions, the experiments were conducted in ultraclean tubes that attained a constant and uniform temperature in a bath. The impedance, which correlates with the drag acting on the quartz friction gauge, was measured for each gas. The authors computed the viscosity of the gas from this drag by using the solution of the Navier-Stokes equation [Kokubun et al., J. Vac. Soc. Jpn. 29, 108 (1986)]. The authors concluded that this study provides a new measurement technique for monitoring the concentration of hydrogen-methane mixed gas. (c) 2007 American Vacuum Society.