  We perform adiabatic regularization of power spectrum in nonminimally coupled
general single-field inflation with varying speed of sound. The subtraction is
performed within the framework of earlier study by Urakawa and Starobinsky
dealing with the canonical inflation. Inspired by Fakir and Unruh's model on
nonminimally coupled chaotic inflation, we find upon imposing near
scale-invariant condition, that the subtraction term exponentially decays with
the number of $ e $-folds. As in the result for the canonical inflation, the
regularized power spectrum tends to the "bare" power spectrum as the Universe
expands during (and even after) inflation. This work justifies the use of the
"bare" power spectrum in standard calculation in the most general context of
slow-roll single-field inflation involving non-minimal coupling and varying
speed of sound.
