PURPOSE: To change the property of carbon into hydrophilic property by providing an electrode in an aq. soln. and dissolving substance constituted of carbon or a carbon such as activated carbon as a main component into this aq. soln. in which current is allowed to flow.

CONSTITUTION: An electrode 2 is made of the substance constituted of carbon on a carbon produced from carbon fiber or activated carbon as a main component. An aq. soln. is introduced into a water tank 1 and the electrode 2 is provided therein and current is allowed to flow. However, the substance besides a substance constituted of carbon as the main component e.g. platinum may be used for the electrode. The substance constituted of carbon or a carbon such as activated carbon as a main component is dissolved into the aq. soln. in a state wherein current is allowed to flow to the electrode. Thereby, carbon particles are formed which have large amounts of acidic radical of strong acid on the surfaces of carbon particles being the main component of carbon or activated carbon. Even when the substance constituted of carbon or the carbon produced from activated carbon as the main component is mixed into the aq. soln. from the first and current is allowed to flow to the electrode, the same result as above-mentioned result is caused. The acidic radicals of strong acid are rapidly formed by blowing reactive gas such as oxygen into the aq. soln.