It is very difficult to estimate the dynamic response of gas flowmeters quantitatively, since the variation of the pressure in an unsteady gas flow is accompanied by the variation of temperature that is quite difficult to measure quantitatively. The isothermal chamber proposed by Tokyo Institute of Technology has a potential to realize a quantitative unsteady gas flow generator because of its ability to keep the gas temperature almost constant during the variation of pressure. AIST is conducting an applied research work to establish an unsteady gas flow generator based on the isothermal chamber. The uncertainty of the instantaneous flow rate in unsteady flows is improved to 5 % by utilizing critical nozzles to define the inlet flow rate into the chamber. The generator shown in the picture can generate arbitrary waveforms by controlling the pressure in the chamber with the maximum instantaneous flow rate of 0.5 g/s at the atmospheric pressure. The flow rate of the generated flow measured by a high-speed laminar flowmeter coincides very well with the control signal fed into the generator.