A memory device that can be highly integrated is provided. The memory device includes a first transistor and a second transistor in a memory cell, and small-area vertical transistors each including a channel formation region on a side surface of an opening portion provided in an insulating layer are used as the two transistors. The memory cell includes a conductor having a function of a gate electrode of the first transistor and a function of one of a source electrode and a drain electrode of the second transistor. The memory cells are placed in a staggered arrangement, so that the memory device can be highly integrated.