In a light modulating apparatus for an excimer discharge lamp high frequency voltage whose frequency is 1000 Hz band or higher is applied to the excimer discharge lamp and light is modulated by changing the frequency of the high frequency voltage, on-time per cycle of the frequency is set so that the on-time is approximately equal to charging time for acquiring electric charges which contribute to light emission of the excimer discharge lamp, whereby stable light modulation is possible. Further, the high frequency voltage is modulated by rectangular waveform voltage having low frequency which is lower than that of the high frequency voltage whereby light can be stably modulated in a wide range.