A low-pressure mercury discharge lamp includes: an arc tube made by winding a glass tube into double spiral configuration; a holding member that is in a form of a cylinder with a closed bottom and is for holding the arc tube; an electronic ballast for lighting the arc tube; a case in a cone form for covering the electronic ballast by being fit to a circumferential wall of the holding member; and a globe covering the arc tube. When an inner diameter of the arc tube is represented as Di mm, and tube wall loading of the arc tube is represented as L W/cm.sup.2, rectangular coordinates (Di, L) are confined within a range defined by points of: (2.5, 0.31), (4.0, 0.35), (5.5, 0.30), (6.5, 0.19), and (2.5, 0.01).