It is an object to provide, in a data storage circuit for storing data, a
power saving data storage circuit and a data writing method in the data
storage circuit, and, further, to provide a data storage device. Thus, in
the present invention, reading out existing data stored in a storage
element M is performed prior to performing writing of new data to the
storage element M to compare the existing data and the new data. The data
storage circuit is configured so that in a case where the existing data
and the new data are identical with each other, writing to the storage
element M is not performed, and, in a case where the existing data and
the new data are not identical with each other, writing of the new data
to the storage element M is performed. The data storage circuit is formed
on a semiconductor substrate to have a data storage device.