  Ultracompact minihalos~(UCMHs) can form after the epoch of matter-radiation
equality, if the density fluctuations of dark matter have significantly large
amplitude on small scales. The constraint on the UCMH abundance allows us to
access such small-scale fluctuations. In this paper, we present that, through
the measurement of 21-cm fluctuations before the epoch of reionization~ we can
obtain a constraint on the UCMH abundance. We calculate the 21-cm signal from
UCMHs and show that UCMHs provide the enhancement of the 21-cm fluctuations. We
also investigate the constraint on the UCMH abundance and small-scale curvature
perturbations. Our results indicate that the upcoming 21-cm observation, the
Square Kilometre Array (SKA), provides the constraint on amplitude of
primordial curvature power spectrum, ${\cal A}_{\zeta} \lesssim 10^{-6}$ on
$100~{\rm Mpc}^{-1} \lesssim k \lesssim 1000~{\rm Mpc}^{-1}$. Although it is
not stronger than the one from the non-detection of gamma rays induced by dark
matter annihilation in UCMHs, the constraint by the SKA will be important
because this constraint is independent of the dark matter particle model.
