  We discuss how one can discriminate models with cold dark matter (CDM) and
baryon isocurvature fluctuations. Although current observations such as cosmic
microwave background (CMB) can severely constrain the fraction of such
isocurvature modes in the total density fluctuations, CMB cannot differentiate
CDM and baryon modes by the shape of its power spectra. However, the evolution
of CDM and baryon density fluctuations are different for each model, thus it
would be possible to discriminate those isocurvature modes by extracting
information on the fluctuations of CDM/baryon itself. We discuss that
observations of 21 cm fluctuations can in principle differentiate these modes
and demonstrate to what extent we can distinguish them with future 21 cm
surveys. We show that, when the isocurvature mode has a large blue-tilted
initial spectrum, 21 cm survey can clearly probe the difference.
