We show that in a quadratic gravity based on Weyl's conformal geometry, the Planck mass scale can be
generated from quantum effects of the gravitational field and the Weyl gauge field via the Coleman-Weinberg
mechanism where a local scale symmetry is broken. At the same time, the Weyl gauge field acquires a mass
less than the Planck mass by absorbing the scalar graviton. The shape of the effective potential is almost flat
owing to a gravitational character and high symmetries, so our model would provide for an attractive model
for the inflationary universe. We also present a toy model showing spontaneous symmetry breakdown of
a global scale symmetry by moving from the Jordan frame to the Einstein one, and point out its problems.