The origin of field-emitted electrons for hydrogen-terminated natural IIb diamond was investigated as a reference to define the origin of field-emitted electrons from lightly nitrogen (N)-doped chemical vapor deposition (CVD) diamond. Using our combined X-ray photoelectron spectroscopy / ultraviolet photoelectron spectroscopy / field emission spectroscopy (XPS/UPS/FES) system, we determined the origin of field-emitted electrons for the natural diamond. The energy level of field-emitted electrons was at valence band maximum (VBM) and the origin of field-emitted electrons was independent of applied voltages. The results suggested the observed shift for FES peak of lightly N-doped CVD diamond is most likely due to the resistance of the diamond bulk. The FES Peak Energy - Emission Current characteristics was best fitted to straight lines, and the resistance of the diamond was consistent with the resistance obtained from the slope. In addition, an attempt was made to define the origin of field-induced electron emission for heavily N-doped CVD diamond. The result suggested a possibility of conduction band minimum (CBM) as an origin of emitted electrons.