Driving condition is determined so that the stimulating values of
brightness with respect to drive data become closer to being regular in
interval. As the drive condition, the frequency of a reference clock used
for the pulse width modulation is changed to be higher than the range of
high gradations in the range of low gradations in drive data.
Accordingly, smaller increment in brightness in the range of low
gradation and larger increment in the range of high gradations are
achieved.