An image display system includes semiconductor light sources that output optical signals including modulated signal portions at timings different from each other, optical fiber cables that respectively transmit the optical signals, a light synthesizer that synthesizes the optical signals to output a synthesized optical signal, a light amount detector that detects an output light amount of the synthesized optical signal, a modulated signal detector that detects the modulated signal portions from the detected output light amount, an image modulator that modulates the synthesized optical signal according to an input image signal to output a modulated image light, and a light source controller that outputs drive control signals for respectively outputting the optical signals respectively including the modulated signal portions according to the image signal, and detects a failure of the optical fiber cables based on whether a modulated signal portion among the plurality of modulated signal portions is detected.