A semiconductor device of this invention is a single-layer gate nonvolatile semiconductor memory in which a floating gate having a predetermined shape is formed on a semiconductor substrate. This floating gate opposes a diffusion layer serving as a control gate via a gate oxide film and is capacitively coupled with the diffusion layer by using the gate oxide film as a dielectric film. The diffusion layer immediately below the dielectric film is insulated from the semiconductor substrate by an insulating film such as a silicon oxide film. A pair of diffusion layers are formed in surface regions of the semiconductor substrate on the two sides of the floating gate extending on a tunnel oxide film. This invention can realize a reliable semiconductor device which is a single-layer gate semiconductor device by which a low-cost process is possible, has a control gate which can well withstand a high voltage applied when data is erased or written, and can prevent an operation error by minimizing variations in the threshold value.