One object is to propose a memory device in which a period in which data is held can be ensured and memory capacity per unit area can be increased. The memory device includes a memory element, a transistor including an oxide semiconductor in an active layer for control of accumulating, holding, and discharging charge in the memory element, and a capacitor connected to the memory element. At least one of a pair of electrodes of the capacitor has a light-blocking property. Further, the memory device includes a light-blocking conductive film or a light-blocking insulating film. The active layer is positioned between the electrode having a light-blocking property and the light-blocking conductive film or the light-blocking insulating film.