To surely illuminate a deuterium discharge tube suitable as light source of a decomposer, such as a spectrophotometer or a liquid chromatograph, and to improve the stability of light volume emitted from the turned-on deuterium discharge tube.

A plurality of resistors 9 and 10 are connected in series with a portion between A and K of a deuterium discharge tube 24. The characteristics of a portion between A and K of the deuterium discharge tube 24 and series circuits of the resistors 9 and 10 seen from power supply are corrected to the positive resistance characteristics in spite of large negative resistance characteristics in glow discharge at the time of discharge starting. Immediately after discharge starts, part of the resistor 10 connected is short-circuited. The comparatively small negative resistance characteristics in glow discharge during continuous discharge are corrected to the positive resistance characteristics by only the resistor 9. Accordingly, the discharge current is stably continued with a proportional control system by negative feedback.