  We investigate how precisely we can determine the nature of dark energy such
as the equation of state (EoS) and its time dependence by using future
observations of 21 cm fluctuations at the epoch of reionization (6.8 <~ z <~
10) such as Square Kilometre Array (SKA) and Omniscope in combination with
those from cosmic microwave background, baryon acoustic oscillation, type Ia
supernovae and direct measurement of the Hubble constant. We consider several
parametrizations for the EoS and find that future 21 cm observations will be
powerful in constraining models of dark energy, especially when its EoS varies
at high redshifts.
