A diamond pn-junction diode electron emitter was examined in an emission spectro-microscope. An electron emission efficiency of ∼10^-2 to the diode current was confirmed at a forward diode current larger than ∼40 μA. Measured electron emission images showed that electron emission indeed occurs at the terrace and up to the terrace boundary of p-type diamond electrode. Measured energy distribution curves of emitted electrons showed that electrons are indeed injected to the conduction band of p-type diamond electrode and emitted from a negative electron affinity surface of p-type diamond. [DOI: 10.1380/ejssnt.2009.660]