A steam generator includes a body defining therein a flow passage having an inlet and an outlet, a heater installed in the body and adapted for heating water being guided through the inlet of the flow passage into the inside of the flow passage into steam for enabling generated steam to flow out of the outlet of the flow passage, and at least one friction unit mounted in the flow passage and movable by generated steam to rub against the inside wall of the flow passage and to remove water scale from the inside wall. Thus, the steam generator uses the friction unit to remove water scale from the inside wall of the flow passage during operation, avoiding water scale accumulation and prolonging the lifespan.