PURPOSE: To enhance the efficiency of light emission, by providing each light- permeable container with an annular discharge groove surrounded by the inner and outer circumferential walls of the container and the radial wall thereof to increase a total discharge length and reduce the absorption of ultraviolet rays performed by mercury atoms.

CONSTITUTION: Each of light-permeable containers 1, 2 comprises an outer and an inner vertical circumferential walls 11, 12 and an annular bottom 10. An annular discharge groove 13 is defined by the walls 11, 12 and the bottom 10 so that discharge is caused in a space comprising the annular discharge grooves 13 and discharge holes 15, 16 of the containers 1, 2. Since the discharge space is curved, the total length of the discharge is made large. Since the discharge is caused near the outer circumferential walls 12 of the containers 1, 2, the ratio of absorption of ultrasonic rays performed by mercury atoms is reduced. The ratio of simulation of coating films 17 of a fluorescent substance on the inside surfaces of the outer circumferential walls 12 is thus augmented to enhance the efficiency of light emission.