An electron cyclotron resonance (ECR) plasma source 16 cm in diameter using a planar ring-cusp magnetic held and a reentrant coaxial cavity was developed. The planar ring-cusp magnetic field produces a large-area ECR surface, and the reentrant coaxial cavity efficiently introduces microwaves into the ECR surface. Large-area, uniform and stable plasmas were generated with a mixture of H-2 + CH4 + O-2 for diamond synthesis. The film deposited by means of the plasma CVD method was made of particles with high-quality crystal habit and with the typical diamond Raman spectrum.