Potential of a word line connected to any selected one of memory cells
is lowered and potential of word lines connected to non-selected memory cells are
raised. The potential of the plate line is raised and lowered. The potential of
the bit line is raised and lowered. After this, reading data from the memory cells
after potential raising and lowering of the plate line and potential raising and
lowering of the bit line have been alternately performed at least one time, thereby
to determine attenuation of polarization in the ferroelectric capacitor.