Then, there is formed a resist pattern 5 for forming a throughhole 7 at a desired portion of the semiconductor device on the way of its manufacturing process, by means of photoengraving process, as shown in FIG. 2C. Further, an isotropic etching is performed to the second plasma oxide film 4 by means of such chemicals as dilute hydrofluoric acid etc. so that a taper portion 6 for reducing the aspe