To read multilevel data from a memory cell having a transistor using silicon and a transistor using an oxide semiconductor, without switching a signal for reading the multilevel data in accordance with the number of the levels of the multilevel data. The electrical charge of a bit line is discharged, the potential of the bit line is charged via a transistor for writing data, and the potential of the bit line which is changed by the charging is read as multilevel data. With such a structure, the potential corresponding to data held in a gate of the transistor can be read by only one-time switching of a signal for reading data.