A deep trench is formed by carrying out etching by using an etching gas free of carbon and silicon, which contains at least one member selected from the group consisting of fluorine, chlorine and bromine, while maintaining an article to be etched at such a temperature that the reaction probability between silicon and fluorine, chlorine or bromine contained in the etching gas is less than 1/10 of the reaction probability at 20.degree. C. According to this method, a deep trench having a very narrow width and a large aspect ratio, which cannot be formed according to the conventional method, can be formed very promptly with much reduced side etching.