A 30 cm-class long electron cyclotron resonance (ECR) plasma source using slot antennas on a rectangular waveguide was developed for large-area processing. Microwaves of 2.45 GHz were radiated efficiently from the slot antennas into a discharge chamber. The plasma source was operated for Ar, O-2 and N-2 at pressures of 0.01-0.2 Pa. The plasma parameters were measured, and the current entering a target located downstream of the plasma source was also measured for sputtering deposition process. The ECR plasma source achieved the production of uniform Ar plasmas above 0.059 Pa. The Ar plasma uniformity was 10% in a 200 mm length at 437 W and 0.094 Pa. The Ar plasma density was beyond the cutoff density, although the plasma densities for O-2 and N-2 were below it. The target current reached about 150 mA for Ar.