  We present observational constraints on a scalar-tensor gravity theory by
$\chi^2$ test for CMB anisotropy spectrum. We compare the WMAP temperature
power spectrum with the harmonic attractor model, in which the scalar field has
its harmonic effective potential with curvature $\beta$ in the Einstein
conformal frame and the theory relaxes toward Einstein gravity with time. We
found that the present value of the scalar coupling, i.e. the present level of
deviation from Einstein gravity $(\alpha_0^2)$, is bounded to be smaller than
$5\times 10^{-4-7\beta}$ ($2\sigma$), and $10^{-2-7\beta}$ ($4\sigma$) for $0<
\beta<0.45$. This constraint is much stronger than the bound from the solar
system experiments for large $\beta$ models, i.e., $\beta> 0.2$ and 0.3 in
$2\sigma$ and $4\sigma$ limits, respectively. Furthermore, within the framework
of this model, the variation of the gravitational constant at the recombination
epoch is constrained as $|G(z=z_{rec})-G_0|/G_0 < 0.05(2\sigma)$, and
$0.23(4\sigma)$.
