When a p-layer 4 composed of GaN is maintained at ordinary temperature and TNO is sputtered thereon by an RF magnetron sputtering method, a laminated TNO layer 5 is in an amorphous state. Then, there is included a step of thermally treating the amorphous TNO layer in a reduced-pressure atmosphere where hydrogen gas is substantially absent to thereby crystallize the TNO layer. At the sputtering, an inert gas is passed through together with oxygen gas, and volume % of the oxygen gas contained in the gas passed through is 0.10 to 0.15%. In this regard, oxygen partial pressure is 5×10−3 Pa or lower. The temperature of the thermal treatment is 500° C. for about 1 hour.