An element isolation region exists at a side opposite to a diffusion layer region as seen from a channel region, without another electrode to which the same potential as one applied to the diffusion layer region is applied interposed between the channel region and the element isolation region. The electric field applied to the gate insulating film is not uniform and the magnitude of the electric field is increased when approaching closer to the diffusion layer region. Therefore, breakdown is likely to occur at parts closer to the diffusion layer region.