Abstract: A method and apparatus for improving the wettability with pure water of the surface of a substrate, in particular, a silicon wafer from which the oxide film has been removed, whereby pure water is brought into contact with the surface of silicon, so that the impurities on the silicon can be completely cleaned away.One feature resides in that microwaves are applied in a state where water is brought into contact with a catalyst. Furthermore, another feature resides in that a catalyst holding means, in which the catalyst is held so as to come in contact with water introduced to the interior thereof, and a microwave producing means for applying the microwaves to the water introduced into the catalyst holding means are provided in the intermediate portion of a piping for delivering the pure water to a use point.