When the threshold voltage Vth of the transistor in the memory cell is within the allowable range is determined, a memory cell which does not have sufficient data retention characteristics is eliminated. In order to eliminate such a memory cell, the potential of a gate of the transistor is kept at an appropriate potential VGM and the potential of a drain of the transistor is set higher than or equal to VGM. When data is written to the memory cell in this state, the potential of a source of the transistor is expressed as a formula including the threshold voltage Vth, (VGM−Vth). By comparison between the level of the potential and the level of a reference potential, whether the threshold voltage Vth is within the allowable range can be determined.