  We explore the capability of measuring lensing signals in $LiteBIRD$ full-sky
polarization maps. With a $30$ arcmin beam width and an impressively low
polarization noise of $2.16\,\mu$K-arcmin, $LiteBIRD$ will be able to measure
the full-sky polarization of the cosmic microwave background (CMB) very
precisely. This unique sensitivity also enables the reconstruction of a nearly
full-sky lensing map using only polarization data, even considering its limited
capability to capture small-scale CMB anisotropies. In this paper, we
investigate the ability to construct a full-sky lensing measurement in the
presence of Galactic foregrounds, finding that several possible biases from
Galactic foregrounds should be negligible after component separation by
harmonic-space internal linear combination. We find that the signal-to-noise
ratio of the lensing is approximately $40$ using only polarization data
measured over $90\%$ of the sky. This achievement is comparable to $Planck$'s
recent lensing measurement with both temperature and polarization and
represents a four-fold improvement over $Planck$'s polarization-only lensing
measurement. The $LiteBIRD$ lensing map will complement the $Planck$ lensing
map and provide several opportunities for cross-correlation science, especially
in the northern hemisphere.
