An object of the present invention is to provide a semiconductor memory device suitable for larger-capacity storage because of its ability to store 3 or more bits in one element and capable of a high-speed and high-efficiency write operation due to a reduced leakage current during the write operation and provide a fabrication method therefor. According to the present invention, each of elements has a source region, a drain region, a control gate, two charge storage regions, and one or more assist gates. During a write operation, source side injection writing is performed with respect to a write target element by using the assist gates, while adjacent elements are isolated by field isolation using the assist gates.