Nonvolatile semiconductor memory device and method of manufacturing the same

In one embodiment, a nonvolatile semiconductor memory device includes a substrate, and a well region formed in the substrate. The device further includes device regions formed in the well region and defined by isolation trenches formed in the well region, the device regions extending in a first direction parallel to a principal surface of the substrate, and being adjacent to one another in a second direction that is perpendicular to the first direction. The device further includes isolation insulators buried in the isolation trenches to isolate the device regions from one another. The device further includes floating gates disposed on the device regions via gate insulators, and a control gate disposed on the floating gates via an intergate insulator. The device further includes first diffusion suppressing layers formed inside the respective device regions to divide each of the device regions into an upper device region and a lower device region. The device further includes second diffusion suppressing layers formed on side surfaces of the respective upper device regions, the side surfaces being perpendicular to the second direction.