  Redshifted 21cm signal is a promising tool to investigate the state of
intergalactic medium (IGM) in the Cosmic Dawn (CD) and Epoch of
Reionization(EoR). In our previous work (Shimabukuro et al 2015), we studied
the variance and skewness to give a clear interpretation of 21cm power spectrum
and found that skewness is a good indicator of the epoch when X-ray heating
becomes effective. Thus, the non-Gaussian feature of the spatial distribution
of the 21cm signal is expected to be useful to investigate the astrophysical
effects in the CD and EoR. In this paper, in order to investigate such a
non-Gaussian feature in more detail, we focus on the bispectrum of the 21cm
signal. It is expected that the 21cm brightness temperature bispectrum is
produced by non-gaussianity due to the various astrophysical effects such as
Wouthysen-Field (WF) effect, X-ray heating and reionization. We study the
various properties of 21cm bispectrum such as scale dependence, shape
dependence and redshift evolution. And also we study the contribution from each
component of 21cm bispectrum. We find that the contribution from each component
has characteristic scale-dependent feature, and it is expected that we could
obtain more detailed information on the IGM in the CD and EoR by using the 21cm
bispectrum in the future experiments, combined with the power spectrum and
skewness.
