To provide a treating device capable of uniformly executing microwave plasma treatment for a substrate of a large area with ≥300 mm diameter at a high speed.

This plasma treating device is composed of a plasma treating chamber 101, a means 103 of supporting the substrate 102 to be treated set in the plasma treating chamber, a means 105 of introducing a gas for treatment into the plasma treating chamber, a means 106 of exhausting the inside of the plasma treating chamber and a means composed of an endless circular waveguide 108 having a plurality of slots and introducing microwaves into the plasma treating chamber, the slots are covered wish dielectric sheets 109 having conductance of >0 to 0.1 l/s, and a gas for treatment is introduced into the plasma treating chamber 101 through the dielectric sheets.