Microwave interference method for the measurement of the electron density of plasmas contained in a metal cylinder is studied theoretically and experimentally. It is pointed out that the interferometer signal through the metal vessel can not eliminate of the cavity mode effect, even if the dimension of the vessel is large compared with the wavelength of the probing microwave. It is concluded that the cavity mode effect can be ignored, if the Q-value of the cavity filled with plasma satisfies the condition Q<2b/λ, where b and λ are the radius of the cylindrical cavity and wavelength of the probing microwave respectively.