An image input system includes a solid state image pickup device and a
preprocessor for performing correlated double sampling amplification on
an output of the image pickup device and outputting a video signal. The
preprocessor has a correlated double sampling amplifier for outputting
signal information corresponding to a difference voltage between the
black level in a feedthrough period of the image pickup device and a
signal level in a charge signal output period; and an offset cancelling
circuit for cancelling an offset voltage corresponding to the difference
voltage in a state where the image pickup device is optically interrupted
to the input terminal of the correlated double sampling amplifier. The
correlated double sampling amplifier cancels out the offset voltage and
the offset cancelling voltage as signal components of polarities opposite
to each other, so that circuits following the correlated double sampling
amplifier are not influenced by the offset voltage.