To provide a backlight device for a liquid crystal panel, providing an auxiliary light source turned on according to a lowering of an ambient temp. of a cold cathode discharge tube and making a beam incident on the liquid crystal panel.

A cold cathode discharge tube 30 is disposed on back side 11 of a liquid crystal panel 10 to make the beam incident on the liquid crystal panel 10, and an auxiliary light source 40 is disposed on the back side 11 of the liquid crystal panel 10 for making the beam incident on the liquid crystal panel 10, and a temp. sensor detects the ambient temp. of the cold cathode discharge tube 30. Then, a microcomputer controls the auxiliary light source unit 40 so as to increase and decrease the emitted light quantity of the auxiliary light source unit 40, according to the lowering and the increase of the detection temp. of the temp. sensor.