  We estimate the efficiency of mitigating the lensing $B$-mode polarization,
the so-called delensing, for the $LiteBIRD$ experiment with multiple external
data sets of lensing-mass tracers. The current best bound on the
tensor-to-scalar ratio, $r$, is limited by lensing rather than Galactic
foregrounds. Delensing will be a critical step to improve sensitivity to $r$ as
measurements of $r$ become more and more limited by lensing. In this paper, we
extend the analysis of the recent $LiteBIRD$ forecast paper to include multiple
mass tracers, i.e., the CMB lensing maps from $LiteBIRD$ and CMB-S4-like
experiment, cosmic infrared background, and galaxy number density from
$Euclid$- and LSST-like survey. We find that multi-tracer delensing will
further improve the constraint on $r$ by about $20\%$. In $LiteBIRD$, the
residual Galactic foregrounds also significantly contribute to uncertainties of
the $B$-modes, and delensing becomes more important if the residual foregrounds
are further reduced by an improved component separation method.
