PURPOSE: To obtain a low pressure mercury lamp which can prevent a blackening of a luminous bulb, and can maintain the ultraviolet-ray output at a high level for a long period, and an ultraviolet-ray radiating device using such a mercury lamp.

CONSTITUTION: While electrodes 4 are sealed at both ends of a luminous tube 2 which consists of a quartz glass, in this low pressure mercury lamp, an electron radiating material is attached to the electrodes, mercury and a rare gas are sealed in the above luminous tube 2, and the lamp current I is made 1A or higher. When the inner surface area of the luminous tube positioning between the electrodes 4 is made S (cm2), the relation of the lamp current I and the inner surface area S is made as S≥50×I+1500. As a result, even though the lamp is lighted for 8500h, the output of the ultraviolet-ray can be maintained 50% or higher.