Non-equilibrium plasma was found to be induced at atmospheric pressure by low-power microwave irradiation of an argon gas stream in a packed bed of activated coke. The effect of microwave power and the presence of N-2 on the behavior of this plasma generation were studied. Plasma density was drastically enhanced over a certain microwave power, but the presence of N-2 could have an inhibitory effect. When the plasma was applied to decomposition of NO contained in argon, sufficiently high conversion of NOx reduction was obtained in the absence of a catalyst or reducing agent. Conversion of NOx reduction was improved only slightly by injection of NH3 as a reducing agent.