A method is disclosed, which is capable of subjecting microorganisms to sterilizing or bacteriostatic treatment with good efficiency as compared with the conventional sterilizing or bacteriostatic method using a noble metal electrode. Microorganism-containing water to be treated is electrochemically treated using an anode having conductive diamond to bring the microorganism into contact with the anode, thereby undergoing sterilization. Since the conductive diamond has a high oxidation potential as compared with other electrode substances, direct oxidation reaction due to contact between a microorganism in water to be treated, such as Legionella bacteria, and the anode surface occurs strongly as compared with other electrodes, thereby enabling effective sterilization. The conductive diamond has a high ability to generate ozone and has an excellent formation efficiency with respect to the generation of hydrogen peroxide and radicals. Accordingly, an indirect sterilizing effect can also be expected.