Provided is a light source device for a lighting fixture, which is prepared with both a reference signal (Vref1) for varying in response to the dimming depths (k1, k2, - - -, and k7) of a dimming signal (k) from a signal value corresponding to the maximum current for a full light to a signal value corresponding to the minimum current for the largest dimming depth, and a reference signal (Vref2) for varying from a signal value corresponding to the load voltage at the time of the maximum current for the full light to a signal value corresponding to the minimum current at the time of the largest dimming depth. In the region near the full light for the small dimming depth, the reference signal (Vref1) is selected to control light emitting diodes (19 to 21) with a constant current in a current control mode. In the region of a large dimming depth, the reference signal (Vref2) is selected to control the light emitting diodes (19 to 21) with a constant voltage in a voltage control mode.