PURPOSE: To efficiently absorb energy of microwave to plasma and to apply the device to a long-sized material without enlarging this device by confining microwave with a multicusp structure and arranging a workpiece to be treated to the advancing direction of microwave.

CONSTITUTION: This plasma treating device 1 is formed of a plasma producing chamber 3 which is partially constituted of a multicusp structure consisting of a plurality of cusps, a treating working chamber 12 and a magnetic field means 18, etc. Microwave is confined with a multicusp structure. An opening 2 is provided to one part of the producing chamber 3 and plasma is generated therein. Further the working chamber 12 is communicated with the plasma producing chamber 3. The workpieces 14 to be treated by plasma are housed parallel to the advancing direction of microwave. Furthermore the magnetic field means 18 pulls out plasma converged by cusps onto the workpieces 14 to be treated via the opening 2.