A plasma irradiation apparatus comprising: a cover housing which is configured to form a predetermined space by partitioning; a plasma generation device which is configured to eject the plasma toward an inner portion of the cover housing; a purge gas supply device which is configured to supply a gas to the inner portion of the cover housing; a stage which is installed in the inner portion of the cover housing and which is for placing a processing target liquid; and a moving device for moving at least one of an ejection port of the plasma generation device from which the plasma is ejected into the inner portion of the cover housing and the stage and arbitrarily changing a distance between the ejection port and the stage, the apparatus is configured to irradiate the processing target liquid which is placed in the inner portion of the cover housing with a plasma; characterized by a control device which is configured to control operations of the plasma generation device and the purge gas supply device, wherein the control device is configured to control operations of the plasma generation device and the purge gas supply device such that a gas is supplied to the inner portion of the cover housing using the purge gas supply device, the plasma is subsequently ejected toward the inner portion of the cover housing using the plasma generation device, and when the plasma is being ejected, the gas is supplied to the inner portion of the cover housing using the purge gas supply device, wherein the plasma irradiation apparatus comprises further a detection sensor which is configured to detect a concentration of a predetermined gas in the inner portion of the cover housing; and a cooling device which is configured to cool the stage. a cover housing which is configured to form a predetermined space by partitioning; a plasma generation device which is configured to eject the plasma toward an inner portion of the cover housing; a purge gas supply device which is configured to supply a gas to the inner portion of the cover housing; a stage which is installed in the inner portion of the cover housing and which is for placing a processing target liquid; and a moving device for moving at least one of an ejection port of the plasma generation device from which the plasma is ejected into the inner portion of the cover housing and the stage and arbitrarily changing a distance between the ejection port and the stage, the apparatus is configured to irradiate the processing target liquid which is placed in the inner portion of the cover housing with a plasma; characterized by a control device which is configured to control operations of the plasma generation device and the purge gas supply device, wherein the control device is configured to control operations of the plasma generation device and the purge gas supply device such that a gas is supplied to the inner portion of the cover housing using the purge gas supply device, the plasma is subsequently ejected toward the inner portion of the cover housing using the plasma generation device, and when the plasma is being ejected, the gas is supplied to the inner portion of the cover housing using the purge gas supply device, wherein the plasma irradiation apparatus comprises further a detection sensor which is configured to detect a concentration of a predetermined gas in the inner portion of the cover housing; and a cooling device which is configured to cool the stage.