We have succeeded in fabricating a diamond p-i-n junction diode. The diode emits high-efficiency deep-ultraviolet (UV) light with the wavelength of shorter than 250 nm by current injection at room temperature. This deep-UV light emission has been realized by using the high density excitonic states in diamond, and can be observed even at 200°C. It is also found that the diode shows high internal quantum efficiency. These results indicate that even indirect transition semiconductors like diamond can emit high-efficiency deep-UV light.