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