A p-GaN semiconductor is the more suitable photocathode material than a p-GaAs semiconductor, because a p-type semiconductor with wide band gap has large vacuum level shift caused by surface band bending. We measured quantum yield degradation and quantum yield spectrum of a p-GaN photocathode with an NEA surface. The p-GaN photocathode had the process of quantum yield degradation from the extraction of photocurrent and the residual gas, it is found to be important to suppress backs-pattering due to the increase of background pressure. From the dependence of quantum yield of the p-GaN photocathode on excitation wavelength, the quantum yield spectrum rapidly increased around the band gap of GaN as expected.