An ignition element for use in igniting a high-frequency plasma torch is ungrounded and displaceable. When a tip end of the ignition element is positioned in a location in a gas to be formed into a plasma, which flows under normal pressure, and a high-frequency energy is applied to the above location in the gas flow, the gas is ignited into a high-temperature plasma in a small period of time shorter than 1 second. After the gas has been ignited, the ignition element is immediately retracted out of the location. The ignition element may be in the form of an ignition rod of metal or an ignition tube of quartz or the like. Where the ignition rod is used, it instantaneously contacts the high-temperature plasma upon ignition so that the high-temperature plasma is of high purity consisting only of the component of the gas. The ignition tube may be used for producing a high-temperature plasma of higher purity on and after ignition. The ignition tube is employed while a pressure therein is reduced. A glow discharge is generated in the ignition tube of the reduced pressure by applying the high-frequency energy, and the gas flowing outside of the tube is ignited by the glow discharge into a plasma.