Reference is made to JP-A-7-335690 which is incorporated herein by reference. [0060] A small diameter third level contact plug 30 a made of conductive material such as W is buried in the small diameter third-level contact hole 28A. This contact plug 30 a is formed by a process similar to that described with reference to FIG. 5 and has the structure similar to that of the first-level contact plug 1