1. A semiconductor device, comprising: a first wiring structure formed on a semiconductor substrate and including a first plug and a first wiring formed on said first plug; and a second wiring structure formed on said semiconductor substrate and including a second plug and a second wiring formed on said second plug; wherein the upper surface of said first wiring is positioned higher than the upper surface of said second wiring, and the lower surface of said first wiring is positioned flush with or lower than the upper surface of said second wiring.