The extraordinary wave (X wave) was found to contribute to the uniformity of an electron cyclotron resonance (ECR) plasma whose electron density is about 1–2×1011 cm-3. The X wave propagated radially in the plasma both before and after an ECR point. The wavelength of the X wave changed radially according to the local electron density. The experimental results indicated a possibility that an upper hybrid resonance (UHR) sustained the very uniform ECR plasma. With increasing input microwave power, the radial profile of the ion saturation current density changed from uniform to convex abruptly, and the electron density at a center increased to about 1012 cm-3 with an electron density jump. Only the whistler wave propagated axially in the high density plasma of about 1012 cm-3. It was found, by measuring electromagnetic waves, that the radial profile of the ion saturation current density abruptly changed when the electron density was nearly equal to the cutoff density of the X wave at the ECR point. © 1998 American Vacuum Society.