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Ambient neutrons are one of the most serious backgrounds for undergroundexperiments in search of rare events. The ambient neutron flux in anunderground laboratory of Kamioka Observatory was measured using a$\mathrm{^3He}$ proportional counter with various moderator setups. Since thedetector response largely depends on the spectral shape, the energy spectra ofthe neutrons transported from the rock to the laboratory were estimated byMonte-Carlo simulations. The ratio of the thermal neutron flux to the totalneutron 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 moderatorwas used to determine this parameter. Consequently, the most-likely neutronspectrum predicted by the simulations for the parameters determined by theexperimental results was obtained. The result suggests an interesting spectralshape, which has not been indicated in previous studies. The total ambientneutron 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 explainour method of the result and discuss our future plan.

Measurement of ambient neutrons in an underground laboratory at Kamioka Observatory and future plan