An atmospheric pressure plasma generator comprising: a pair of electrodes (24, 26) for generating plasma by discharge; a reaction chamber incorporating the pair of electrodes (24, 26) and having an inlet port through which a process gas flows in and an outlet port (39, 51) through which a plasma gas flows out as a result of the process gas being plasmatized by the pair of electrodes (24, 26), and a control device for controlling a temperature of the plasma gas flowing out from the outlet port (39, 51) so that an amount of generation of NOx becomes a threshold or smaller characterized in that the atmospheric pressure plasma generator comprises at least one of the following configurations A) to C): A) the atmospheric pressure plasma generator comprises a NOx sensor disposed in a flow path of the plasma gas, wherein the control device performs the control based on an output from the NOx sensor; B) the plasma gas is an oxygen plasma gas, the atmospheric pressure plasma generator comprises an ozone sensor disposed in the flow path of the plasma gas, and the control device controls a temperature of the plasma gas based on an output from the ozone sensor so that an amount of generation of ozone falls in a range of a predetermined value or greater; C) the atmospheric pressure plasma generator comprises a temperature sensor disposed in the flow path of the plasma gas, wherein the control device controls so that a temperature based on an output from the temperature sensor becomes a target temperature. a pair of electrodes (24, 26) for generating plasma by discharge; a reaction chamber incorporating the pair of electrodes (24, 26) and having an inlet port through which a process gas flows in and an outlet port (39, 51) through which a plasma gas flows out as a result of the process gas being plasmatized by the pair of electrodes (24, 26), and a control device for controlling a temperature of the plasma gas flowing out from the outlet port (39, 51) so that an amount of generation of NOx becomes a threshold or smaller characterized in that the atmospheric pressure plasma generator comprises at least one of the following configurations A) to C): A) the atmospheric pressure plasma generator comprises a NOx sensor disposed in a flow path of the plasma gas, wherein the control device performs the control based on an output from the NOx sensor; B) the plasma gas is an oxygen plasma gas, the atmospheric pressure plasma generator comprises an ozone sensor disposed in the flow path of the plasma gas, and the control device controls a temperature of the plasma gas based on an output from the ozone sensor so that an amount of generation of ozone falls in a range of a predetermined value or greater; C) the atmospheric pressure plasma generator comprises a temperature sensor disposed in the flow path of the plasma gas, wherein the control device controls so that a temperature based on an output from the temperature sensor becomes a target temperature. A) the atmospheric pressure plasma generator comprises a NOx sensor disposed in a flow path of the plasma gas, wherein the control device performs the control based on an output from the NOx sensor; B) the plasma gas is an oxygen plasma gas, the atmospheric pressure plasma generator comprises an ozone sensor disposed in the flow path of the plasma gas, and the control device controls a temperature of the plasma gas based on an output from the ozone sensor so that an amount of generation of ozone falls in a range of a predetermined value or greater; the plasma gas is an oxygen plasma gas, the atmospheric pressure plasma generator comprises an ozone sensor disposed in the flow path of the plasma gas, and the control device controls a temperature of the plasma gas based on an output from the ozone sensor so that an amount of generation of ozone falls in a range of a predetermined value or greater; C) the atmospheric pressure plasma generator comprises a temperature sensor disposed in the flow path of the plasma gas, wherein the control device controls so that a temperature based on an output from the temperature sensor becomes a target temperature.