A semiconductor device having a memory cell region comprising a plurality of memory cells is described, and a stable characteristic is imparted to all the memory cells provided in the memory cell block. Impurities are implanted into a memory cell region of a silicon substrate at predetermined intervals, thus forming a plurality of wells. A resist film used as a mask for implanting impurities has strip-shaped patterns and a broad pattern. Since the strip-shaped patterns located close to the broad pattern are inclined, the characteristics of the wells located in the vicinity of the outer periphery of the memory cell region become unstable. The wells having unstable characteristics are taken as dummy wells which do not affect the function of a semiconductor device.