A novel liquid crystal display device is provided.Two pixels each include a transistor that includes a back gate. The transistor is connected to the common gate line and data line. A threshold voltage is controlled by a control signal supplied to a back gate so that while data is written to one pixel, the transistor of the other pixel is not turned on. The aperture ratio is improved by reducing the number of wirings connected to the pixel and increasing the frequency of the control signal supplied to the back gate.