A light source device for use in image readout devices capable of light emission in which a dielectric barrier discharge fluorescent lamp is synchronized with an external synchronization signal without attendant fluctuation in optical power. A phase comparator compares the oscillation signal phase of a variable frequency oscillator divided by a frequency divider with an external synchronization signal. The phase comparator controls the oscillation frequency of a variable frequency oscillator as a function of the phase difference. Consequently, the oscillation phase of the variable frequency oscillator is phase locked by the external synchronization signal. The oscillation signal of the variable frequency oscillator is input to the gate signal generation circuit, and the switch devices of an inverter circuit are opened and closed by the output of a gate signal generation circuit. The direct current voltage output by a DC power source is converted into alternating current voltage and is output by an inverter circuit and is applied to a lamp via a boosting transformer in order to light the lamp.