A memory element capable of operating at high speed and reducing power consumption and a signal processing circuit including the memory element are provided. As a writing transistor, a transistor which is formed using an oxide semiconductor and has significantly high off-state resistance is used. In a memory element in which a source of the writing transistor is connected to an input terminal of an inverter, a control terminal of a transfer gate, or the like, the threshold voltage of the writing transistor is lower than a low-level potential. The highest potential of a gate of the writing transistor can be a high-level potential. When the potential of data is the high-level potential, there is no potential difference between a channel and the gate; thus, even when the writing transistor is subsequently turned off, a potential on the source side hardly changes.