PURPOSE: To construct a device concerned in a small size and suppress the cost by forming a pair of hot cathodes of a fluorescent discharge lamp provided with different lifetimes, and installing a thermosensitive element only at the tube end side fitted with that of the hot cathodes which is set on the shorter lifetime side.

CONSTITUTION: A fluorescent discharge lamp 1 is equipped with a pair of hot cathodes 2. 3, which are provided with different lifetimes, and a temp. fuse 4 as a thermosensitive element is installed only on the side with that of the hot cathodes 2 which is formed with shorter lifetime. Accordingly the lifetime of the lamp 1 depends upon the hot cathode 2 set with shorter lifetime and can be notified certainly through provision of thermosensitive element on that of the tube ends where this hot cathode 2 is installed. Thereby the number of thermosensitive elements can be decreased to the half to lead to constructing of the device in small size and suppressing the cost.