Plasma generated in water vapor bubbles present in a water-containing liquid is brought into contact, in the liquid, with an article having a contact angle with water of 90.degree. or less. The plasma is contacted with an organic substance adhering to the article to thereby remove the organic substance from the article. By bringing the plasma into contact with the article, the surface of the article is etched without breaking the article. The article may comprise a material composed of both a hydrophobic part having a contact angle with water exceeding 90.degree. and a hydrophilic part having a contact angle with water of 90.degree. or less. In this case only the hydrophobic part is etched by bringing the plasma into contact with the article.