A transistor includes first and second control gates, and a storage gate. The storage gate is made to be a conductor, supplied with a specific potential, and then made to be an insulator, thereby holding the potential. Data is written by making the storage gate a conductor, supplying a potential of data to be stored, and making the storage gate an insulator. Data is read by making the storage gate an insulator, supplying a potential to a read signal line connected to one of a source and a drain of the transistor, supplying a potential for reading data to the first control gate, and then detecting a potential of a bit line connected to the other of the source and the drain.