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 [O III] 88 mu m, [CII] 158 mu m, and dust continuum emission during the epoch of reionization. Our targets are the [N II] 122 mu m fine-structure emission line and the underlying 120 mu m dust continuum. The dust continuum is detected with a similar to 19 sigma significance. From far-infrared spectral energy distribution sampled at 90, 120, and 160 mu m, we obtain a best-fit dust temperature of 40 K (79 K) and an infrared luminosity of log(10)(L-IR/L-circle dot)=11.6 0.4 Z(circle dot)), the ionization parameter and hydrogen density are log(10)U similar or equal to-2.7 +/- 0.1 n(H) similar or equal to 50-250 cm(-3), respectively, which are comparable to those measured in low-redshift galaxies. The nitrogen-to-oxygen abundance ratio, N/O, is constrained to be subsolar. At Z < 0.4 Z(circle dot), the allowed U drastically increases as the assumed metallicity decreases. For high ionization parameters, the 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.