  We report on the emission of terahertz radiation by irradiation of
femtosecond laser pulses in non-centrosymmetric paraelectric and ferroelectric
phases of Co$_3$B$_7$O$_{13}$I boracite. The Generation of the terahertz waves
in both phases is caused by optical rectification via a second-order nonlinear
optical effect. In the ferroelectric phase, we successfully visualized
ferroelectric domains by analyzing the polarization state of the terahertz wave
radiated from the crystal. In a large area of the crystal ($\sim$ 500 $\times$
500 $\mu$m$^2$), the observed polarization vector of the radiated terahertz
wave was tilted from directions of spontaneous polarization, i.e., [100]$_{\rm
cub}$, [010]$_{\rm cub}$, and [001]$_{\rm cub}$ in cubic setting, which can be
explained by the presence of a ferroelectric 90$^\circ$ domain wall of the
(101)$_{\rm cub}$ plane.
