A zero-crossing detecting device that detects a zero-crossing point of AC voltage, the device has a full-wave rectifier that rectifies the AC voltage and outputs a full-wave rectified voltage, a charger that is charged at a predetermined charging voltage by application of the full-wave rectified voltage, wherein the charger outputs a charging current when the full-wave rectified voltage falls below the charging voltage, and a signal output part that outputs a zero-crossing detecting signal. The signal output part outputs the zero-crossing detecting signal when the charging current flows to the signal output part.