To obtain an electrodeless discharge lamp with its high ultraviolet-ray radiation efficiency capable of obtaining stable ultraviolet-ray output by controlling the temperature at the coolest portion of an envelope of the electrodeless discharge lamp and holding the coolest portion within a specified temperature range.

A thermoelectric heat insulation member 7 is attached at a tip part of an outer periphery part 2 which is most distant from an energizing coil 3 wound around the envelope of an electrodeless discharge lamp 1 to form the coolest part. A heater wire is buried in the thermoelectric heat insulation member 7, and the heater wire is heated by means of a power source 8. A temperature of the thermoelectric heat insulation member 7 is detected by means of a temperature detection part 9 and controlled so as to be within a specified temperature range by means of a control part 10. Thus, the coolest portion is held within the specified temperature range, and whereby the electrodeless discharge lamp 1 with its high ultraviolet-ray radiation efficiency capable of having a stable ultraviolet-ray output is obtained.