To achieve an illumination system capable of detecting a zero cross of an AC voltage with accuracy and performing dimming by stable phase control based on an obtained zero-cross signal.

A zero-cross detector 14 detects a zero cross of a low-distortion AC voltage immediately after conversion into a low voltage from a secondary side 12b of a step-down transformer 12. The detection result is generated as a zero-cross signal. The zero-cross signal is transmitted to a dimming control part 13 side dimming the brightness of a light source via a transmitter 15 wirelessly. The dimming control part 13, based on the timing of the zero-cross signal, outputs from a controller 19 signals for performing phase control of an AC voltage which are inputted to dimmers 211-21n corresponding to a dimming desk 20 by using a dimming signal from the dimming desk 20 to perform dimming of light sources 221-22n connected to outputs of the dimmers 211-21n respectively.