Output signals Vp from a density sensor are sampled for plural positions on
an intermediate transfer belt and an amount of toner adhesion is determined based
on the results of sampling. Since these sample data pieces may contain noises,
each predetermined number of data pieces of higher order and of lower order are
removed from the resultant sample data string. The removed data pieces are each
replaced by an average value Vpavg of the other sample data pieces. The amount
of toner adhesion is calculated based on the data string thus replaced.
