A semiconductor device includes a transistor including a first gate (front gate) and a second gate (back gate) overlapping with each other with a semiconductor film therebetween, and a display element. The transistor and the display element are electrically connected to each other. The first gate is supplied with one of 2N−k potentials. The second gate is supplied with one of 2k potentials. One of 2N−k potentials and one of 2k potentials are obtained in such a manner that N-bit digital data is divided into (N−k)-bit digital data and k-bit digital data and they are subjected to D/A conversion. At this time, the total number of gray level power supply lines used for D/A conversion is equal to the sum of 2N−k and 2k. This is smaller than 2N, which the total number of the gray level power supply lines usually needed for D/A conversion of N-bit digital data.