Low temperature plasma is known as a key technology in variety of applications like renewable energy, environmental technology, large flat-screen display, semiconductor process, and so on. Especially, as the plasma does not leave any waste for generating, it plays a very important role in purification of green-house gases and polluting gases. So, low temperature plasma is thought as a leading technology in the future environmental technology. Environmental technology by gas treatment is one of the low temperature plasma applications that handles green-house gases and polluting gases by discharging it with plasma. To ignite and maintain such plasma, a power supply must be able to provide high operation frequency and high output power. But, as the operation frequency and the output power level of a switching converter increase, parasitic components oscillate with each other at switching transients, and the oscillation severely degrades the system reliability by causing malfunction of switching devices. This study has proposed a switching power supply with high reliability by reducing parasitic oscillation. A prototype of the proposed switching power supply has been tested and demonstrated to provide high reliability at 400kHz of high operation frequency and 8.6-kVA of high imaginary output power.