PURPOSE: To make it possible to realize a variable color light source having a miniature size and practically usable brightness by installing a plurality of back electrodes, and controlling the potentials of respective back electrodes to shift their electric discharge fields for modulation.

CONSTITUTION: A plurality of respective back electrodes 111, 112, 113 are installed and the potentials of respective back electrodes are controlled to shift their electric discharge fields so that subsequent modulation is carried out. Phosphor 121 having a red radiation color, phosphor 122 having a green radiation color and phosphor 123 having a blue radiation color are arranged in the portions corresponding to the areas above respective electrodes in a fluorescent tube in relation to the back electrodes 111, 112, 113 of said fluorescent tube. When the potential of an electrode (a) 111 is fixed to an appropriate value, an electric discharge field in the fluorescent tube is fixed to a part above this electrode indicated by a mark of X. Ultraviolet rays are isotropically emitted from this electric discharge field and then most of them enter said red phosphor 121 being close thereto so as to cause red radiation of the fluorescent tube. As a result of this, a light source which has a miniature size, sufficient brightness and capability of varying radiation colors can be realized.