It is an object to reduce power consumption of a 2Tr1C type semiconductor memory device. The absolute value of the threshold voltage of a reading transistor is made larger than a fluctuation range of a data potential of a bit line (or the fluctuation range of the data potential of the bit line is made smaller than the absolute value of the threshold voltage of the reading transistor), whereby the potential of a source line can be fixed, a fluctuation in a potential of a writing word line can be made smaller, and a potential of a reading word line is fluctuated only at the time of reading. Further, a gate of such a transistor the absolute value of the threshold voltage of which is large is formed using a material having a high work function, such as indium nitride.