  We study the effective potential in renormalizable quantum gravity with a
single dimensionless conformal coupling without a Landau pole. In order to
describe a background-free dynamics at the Planck scale and beyond, the
conformal-factor field is quantized exactly in a nonperturbative manner. Since
this field does not receive renormalization, the field-independent constant in
the effective potential becomes itself invariant under the renormalization
group flow. That is to say, it gives the physical cosmological constant. We
explicitly calculate the physical cosmological constant at the one-loop level
in the Landau gauge. We find that it is given by a function of renormalized
quantities of the cosmological constant, the Planck mass and the coupling
constant, and it should be the observed value. It will give a new perspective
on the cosmological constant problem free from an ultraviolet cutoff.
