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Ultracompact minihalos~(UCMHs) can form after the epoch of matter-radiationequality, if the density fluctuations of dark matter have significantly largeamplitude on small scales. The constraint on the UCMH abundance allows us toaccess such small-scale fluctuations. In this paper, we present that, throughthe measurement of 21-cm fluctuations before the epoch of reionization~ we canobtain a constraint on the UCMH abundance. We calculate the 21-cm signal fromUCMHs and show that UCMHs provide the enhancement of the 21-cm fluctuations. Wealso investigate the constraint on the UCMH abundance and small-scale curvatureperturbations. Our results indicate that the upcoming 21-cm observation, theSquare Kilometre Array (SKA), provides the constraint on amplitude ofprimordial curvature power spectrum, ${\cal A}_{\zeta} \lesssim 10^{-6}$ on$100~{\rm Mpc}^{-1} \lesssim k \lesssim 1000~{\rm Mpc}^{-1}$. Although it isnot stronger than the one from the non-detection of gamma rays induced by darkmatter annihilation in UCMHs, the constraint by the SKA will be importantbecause this constraint is independent of the dark matter particle model.

The 21-cm signals from ultracompact minihalos as a probe of primordial small-scale fluctuations