A non-volatile semiconductor memory device includes a semiconductor substrate, an insulating film formed on the semiconductor substrate, a plurality of memory cells formed on the semiconductor substrate, a plurality of first assist gates extending toward the memory cell, a connection portion connecting end portions of the first assist gates, a second assist gate extending toward the memory cell, a first select transistor controlling whether to apply a voltage to an area under the first assist gate, a second select transistor controlling whether to apply a voltage to an area under the second assist gate, and an impurity region. The insulating film formed under an intersection area of the connection portion and the impurity region has a thickness larger than the insulating film formed under the first and second assist gates. A non-volatile semiconductor memory device capable of ensuring a writing speed as well as reliability can thus be obtained.