It is not so easy to measure a flow rate of compressible fluids directly, because not only the pressure but also the temperature must be measured. In this paper, a chamber called an “Isothermal Chamber” is proposed. Then, a simple method to measure compressible fluids using this chamber is proposed. The isothermal chamber is a chamber that can almost realize isothermal conditions due to the larger heat transfer area and heat transfer coefficient by stuffing the steel wool into it. As the state during charge or discharge is almost isothermal, an instantaneous flow rate could be obtained using only a pressure in the chamber. At first, the characteristic of the isothermal chamber was examined by experiments and simulations. Then, the steady flow rate of air was measured by the proposed method. To confirm the effectiveness of the method, the measured results were compared with the results obtained from the sonic venturi nozzle. It became clear that the proposed method can measure the steady flow rate of air within 1 percent of errors.