A power supply for illumination includes a detection circuit and a control circuit. The detection circuit compares an AC voltage whose phase is controlled with a first threshold voltage so as to detect a variation in a conduction state of phase control in the AC voltage, and compares the AC voltage with a second threshold voltage lower than the first threshold voltage so as to detect a zero-cross point of the AC voltage, thereby detecting a conduction period of the phase control. The control circuit outputs an output current according to the duration of the conduction period.