An apparatus for processing with plasma wherein a gas is introduced into a plasma forming chamber (20) through a gas introduction pipe (22), input microwave from a microwave source (31) is supplied to the plasma forming chamber (20), and the gas introduced is converted into a plasma by electron cyclotron resonance. The input microwave in TE mode from a microwave source (31) is received by a tapered waveguide (60) which accomodates a dielectric board (63), and at least part of the input microwave is converted into a microwave portion in EH mode having an electric field component in the traveling direction of the input microwave. A hybrid-mode microwave containing the waves in these two modes is guided into the plasma forming chamber (20) via a microwave introduction window (27). Since the plasma forming chamber (20) receives the hybrid-mode wave of a propagation mode having an electric field component toward the traveling direction, i.e., having a longitudinal component, the microwave energy is efficiently supplied to the plasma region that satisfies the ECR condition, and absorbed in the plasma. Therefore, the plasma forming efficiency increases and the throughput of processing with plasma increases, too.