The characteristics of argon plasma discharged by electron-cyclotron resonance (ECR) with the microwave frequencies of 7.0, 8.0, and 9.4 GHz are investigated in the pressure range of 0.013 -0.13 Pa, where the magnetic field was adjusted to keep the ECR point at the same position. The input power of the microwave was set at 10 and 50 W. Plasma densities and the electron temperatures were found to be almost independent of the microwave frequency at low pressure. The dependence of the electron density on the microwave frequencies becomes apparent in the higher-pressure range above 0.080 Pa. In the case that the discharge power is 50 W, the plasma operated using 9.4 GHz microwave showed the highest electron density and the plasma operated using 7.0 GHz had the lowest electron density. Metastable-atom densities were estimated by using a collisional radiative model. The mestable-atom densities, similar to 10(17) m(-3), are lower than the electron density similar to 10(18) m(-3) at the discharge power of 50 W, whereas these densities were comparable at the discharge power of 10 W.