There are provided the steps of: forming a connection hole in an interlayer insulating film overlying a lower metal interconnection; forming a W plug in the connection hole; forming a first metal film and a second metal film over the interlayer insulating film and the W plug; forming an interconnection underlying film by using a photoresist mask with no alignment margin; and forming a diffusion preventing film made of a titanium fluoride or the like over the W plug, while etching away the exposed part of the first metal film. Reciprocal diffusion of tungsten and aluminum is prevented by the titanium fluoride or the like, thereby preventing the formation of an alloy having high electric resistivity. As a result, an alloy having high electric resistivity resulting from the reaction between the metal plug and the upper metal interconnection is prevented from being formed and a semiconductor device which is high in reliability and integration is provided through the manufacturing process involving no alignment margin.