Semiconductor device with dummy gates in peripheral region

According to one embodiment, a semiconductor device includes a memory region and a peripheral circuit region, the peripheral circuit region includes a first region and a second region outside of the first region. The semiconductor device includes, in the first region, a transistor including a gate insulating layer and a gate structure that includes a gate electrode. A first structure is in the second region and includes a first insulating layer and a dummy gate electrode on the first insulating layer. The first insulating layer has a side surface facing outward from the peripheral circuit region and a second insulating layer that covers the first side surface and is an insulating material other than a silicon oxide.