A semiconductor device which can retain data for a long period is provided. The semiconductor device includes a memory circuit and a retention circuit. The memory circuit includes a first transistor and the retention circuit includes a second transistor. The memory circuit is configured to write data by turning on the first transistor and to retain the data by turning off the first transistor. The retention circuit is configured to supply a first potential at which the first transistor is turned off to a back gate of the first transistor by turning on the second transistor and to retain the first potential by turning off the second transistor. The first transistor and the second transistor have different electrical characteristics.