[Problem] To provide a device and a method for oxidation decomposition treatment of a hazardous gas of a volatile organic compound (VOC) or the like at normal temperature. [Solution] A gas treatment device characterized in being provided with a plasma generating unit and a catalyst medium. The plasma generating unit is provided with at least a flow channel through which a gas to be treated flows； and a power supply unit for supplying electrical power a first electrode a second electrode and a dielectric material arranged inside the flow channel. A voltage is impressed between the first electrode and the second electrode by the power supply unit and electrical discharging is caused to occur whereby plasma is generated. The catalyst medium is adapted for accelerating a reaction with the gas to be treated and is provided in a position where the plasma generated by the plasma generating unit inside the flow channel is present wherein the catalyst medium has metallic catalytic particles present on an inorganic substance.
