A gas treatment device includes 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, and the catalyst medium has metallic catalytic particles present on an inorganic substance.