  Ambient neutrons are one of the most serious backgrounds for underground
experiments in search of rare events. The ambient neutron flux in an
underground laboratory of Kamioka Observatory was measured using a
$\mathrm{^3He}$ proportional counter with various moderator setups. Since the
detector response largely depends on the spectral shape, the energy spectra of
the neutrons transported from the rock to the laboratory were estimated by
Monte-Carlo simulations. The ratio of the thermal neutron flux to the total
neutron flux was found to depend on the thermalizing efficiency of the rock.
Thus, the ratio of the count rate without a moderator to that with a moderator
was used to determine this parameter. Consequently, the most-likely neutron
spectrum predicted by the simulations for the parameters determined by the
experimental results was obtained. The result suggests an interesting spectral
shape, which has not been indicated in previous studies. The total ambient
neutron flux is $(23.5 \pm 0.7 \ \mathrm{_{stat.}} ^{+1.9}_{-2.1} \
\mathrm{_{sys.}}) \times 10^{-6}$ cm$^{-2}$ s$^{-1}$. In this paper, we explain
our method of the result and discuss our future plan.
