There is provided a water treating method which can not only improve an effect of eliminating microorganisms in water intended for cooking and drinking or waste water but also prevent a reduction in trace amounts of nutritional elements. In the method, at least a pair of carbon fibers capable of collecting at least microorganisms are immersed in for-treatment water, potentials of opposite polarities are applied to the carbon fibers, and the polarities of the potentials are switched at a predetermined time interval so as to adsorb the microorganisms on the carbon fibers. After the microorganisms are adsorbed, a positive potential is applied to one of the carbon fibers, and a negative potential is applied to the other so as to cause electrolysis. Then, an alternating voltage is applied to the carbon fibers so as to heat the for-treatment water and the carbon fibers.