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If the symmetry breaking inducing the axion occurs after the inflation, thelarge axion isocurvature perturbations can arise due to a different axionamplitude in each causally disconnected patch. This causes the enhancement ofthe small-scale density fluctuations which can significantly affect theevolution of structure formation. The epoch of the small halo formation becomesearlier and we estimate the abundance of those minihalos which can host theneutral hydrogen atoms to result in the 21cm fluctuation signals. We find thatthe future radio telescopes, such as the SKA, can put the axion mass bound oforder $m_a \gtrsim 10^{-13}$ eV for the simple temperature-independent axionmass model, and the bound can be extended to of order $m_a \gtrsim 10^{-8}$eVfor a temperature-dependent axion mass.

Probing axion dark matter with 21cm fluctuations from minihalos