A pulse voltage is applied on a process gas to generate discharge plasma. The pulse voltage has a duty ratio controlled in a range of 0.001 percent or more and 8.0 percent or less. Preferably, the discharge plasma has an electron density of 1.times.10.sup.10 cm.sup.-3 or larger and an electron temperature of 1.5 eV or lower at a supplied power of 1.0 W/cm.sup.2 or more per a unit area of a discharge electrode.