To allow a large amt. of exciting active seed to arrive at the surface to be treated and to execute treatment with high efficiency at high speed by forming exciting active seed of gas by gas discharge, ejecting a treating gas contg. this exciting active seed onto the object to be treated and simultaneously reducing a boundary layer on the surface of the object to be treated.

While discharge gas is fed from a gas feeding source, high-frequency voltage is applied to a space between electrodes, and gas discharge is generated within a gas passage 12 to form plasma. The action of this plasma generates the exciting active seed of gas. The treating gas contg. this exciting active seed is fed from the gas passage 12 to an intermediate chamber 21, is passed through a supersonic generator 22 and is ejected onto the surface of the object 17 to be treated from a gas ejecting nozzle 20. When the treating gas entered the intermediate chamber 21 passes through the air passage 23 of the supersonic generator 22 at high speed, the oscillation of supersonic frequency is generated. In this way, the treating gas is ejected as a pulsating wave gas stream. On the surface of the object 17 to be treated, the boundary layer of the treating gas is destroyed by the oscillation, and surface-treatment is executed at high speed.