We present new Atacama Large Millimeter/submillimeter Array Band 7 observational results of a Lyman break galaxy at $ z=7.15 $, B14-65666 (Big Three Dragons), which is an object detected in [OIII] 88 $rm{mu m}$, [CII] 158 $rm{mu m}$, and dust-continuum emission during the epoch of reionization. Our targets are the [NII] 122 $rm{mu m}$ fine-structure emission line and underlying 120 $rm{mu m}$ dust continuum. The dust continuum is detected with a $ sim $19$ sigma $ significance. From far-infrared spectral energy distribution sampled at 90, 120, and 160 $rm{mu m}$, we obtaine a best-fit dust temperature of $ 40 $ K ($ 79 $ K) and an infrared luminosity of $ log_{10}(L_{rm IR}/{rm L}_odot)=11.6$ ($12.1$) at the emissivity index $ beta = 2.0 $ (1.0). The [NII] 122 $rm{mu m}$ line is not detected. The 3$ sigma $ upper limit of the [NII] luminosity is $ 8.1 times 10^7 {rm L}_odot$. From the [NII], [OIII], and [CII] line luminosities, we use the Cloudy photoionization code to estimate nebular parameters as functions of metallicity. If the metallicity of the galaxy is high ($ Z > 0.4 {rm Z}_odot$), the ionization parameter and hydrogen density are $ log_{10} U simeq -2.7pm0.1$ and $ n_text{H} simeq 50$-$250 {rm cm}^{-3}$, respectively, which are comparable to those measured in low-redshift galaxies. The nitrogen-to-oxygen abundance ratio, $rm{N/O}$, is constrained to be sub-solar. At $ Z < 0.4 {rm Z}_odot$, the allowed $ U $ drastically increases as the assumed metallicity decreases. For high ionization parameters, the $rm{N/O}$ constraint becomes weak. Finally, our Cloudy models predict the location of B14-65666 on the BPT diagram, thereby allowing a comparison with low-redshift galaxies.
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