  The cosmic microwave background (CMB) observation by the Planck satellite
precisely determines primordial curvature fluctuations on larger scales than
$\mathcal O(1)\,\mathrm{Mpc}$, while the small-scale curvature fluctuation is
still less constrained. The constraint on small-scale fluctuations is highly
improved if we assume the standard thermal relic dark matter scenario. When
small-scale fluctuations are large enough, dense regions collapse to form small
halos even in a redshift $z\gtrsim10^3$, which is called ultracompact
minihalos. These minihalos enhance the annihilation of the dark matter and it
is constrained by observations such as extragalactic gamma rays and the CMB. We
revisit the effect of minihalos formed by the small-scale density fluctuations
and calculate the ionization history modified by the dark matter annihilation.
We perform the Markov Chain Monte Carlo method to constrain the size of
small-scale curvature fluctuations by the CMB power spectrum. It is found that
the constraint from the CMB power spectrum is comparable to that from the
extragalactic gamma rays. We confirm that our constraint mainly comes from the
energy injection in early time ($z\gtrsim 100$) and hence it is independent of
the uncertainty of minihalo properties in the late time.
