In an atmospheric pressure plasma irradiation apparatus 10 of the present invention, an upper cover 76 is lowered and a lower cover 78 is sealed to seal a cover housing 22. After an inert gas is supplied to an inner portion of the cover housing, a plasma heading toward the inner portion of the cover housing is ejected by a plasma generation device 20. Even during the ejection of the plasma, the supplying of the inert gas to the inner portion of the cover housing is continued. Accordingly, it becomes possible to vent air using the inert gas from the inner portion of the cover housing and it becomes possible to manage an oxygen concentration of a periphery of a processing target object which serves as a target of plasma irradiation.
