  The general structure of the conformal anomaly and the dilaton's effect to it
are analysed. First we give a new formal proof of the statement that the
conformal anomaly, in the theory which is conformal invariant at the classical
level, is conformal invariant. The heat-kernel regularization and Fujikawa's
method are taken for the analysis. We present a new explicit result of the
conformal anomaly in 4 dimensional photon-dilaton gravity. This result is
examined from the point of the general structure.
