A steam generator for a washing machine comprises: a hermetic container provided with a water supply port for supplying water and a steam exhaustion port for exhausting steam; a heater arranged in the hermetic container for heating water supplied into the hermetic container; and a drain unit for draining residual water inside of the hermetic container outwardly. According to this, residual water inside of the steam generator is outwardly exhausted thus to maintain cleanness of the steam generator. Also, the drain unit using a siphon principle is installed thus to naturally exhaust residual water inside of the steam generator without an additional valve device, thereby reducing a fabrication cost.