Diamond has various advantages as an electron emitter in addition to the low-threshold voltage, negative electron affinity (NEA), high thermal conductivity, high mechanical hardness, and high chemical stability. The difficulty in clarification of electron emission mechanism is preventing diamond from being used in the practical use. It is extremely difficult to identify the surface potential of the emitting diamond from conventional Emission current (I)- Anode voltage (V) characteristics. If one could measure the potential of the emitting surface, the band diagram of emitting diamond can be completed. The combined spectroscopy of XPS/UPS/FES specially built for this study, is one of the most powerful tool, which could identify the potential of the emitting surface. In this study, we have succeeded in observing the origin of field-emitted electrons using our combined XPS/UPS/FES system.