We have collected one-dimensional raster-scan observations of the active star-forming region Sharpless 171 (S171), a typical H region-molecular cloud complex, with the three spectrometers (LWS, SWS, and PHT-S) on board ISO. We have detected 8 far-infrared fine-structure lines, (O )5 2µm, (N )5 7µm, (O )6 3µm, (O )8 8µm, (N ) 122 µm, (O ) 146 µm, (C ) 158 µm, and (Si )3 5µm together with the far-infrared continuum and the H2 pure rotation transition (J = 5-3) line at 9.66 µm. The physical properties of each of the three phases detected, highly-ionized, lowly-ionized and neutral, are investi- gated through the far-infrared line and continuum emission. Toward the molecular region, strong (O ) 146 µm emission was observed and the (O )6 3µm to 146 µm line ratio was found to be too small (∼5) compared to the values predicted by current photodissociation region (PDR) models. We examine possible mechanisms to account for the small line ratio and conclude that the absorption of the (O )6 3µ ma nd the (C) 158 µm emission by overlapping PDRs along the line of sight can account for the observations and that the (O ) 146 µm emission is the best diagnostic line for PDRs. We propose a method to estimate the effect of overlapping clouds using the far-infrared continuum intensity and derive the physical properties of the PDR. The (Si )3 5µm emission is quite strong at almost all the observed positions. The correlation with (N ) 122 µm suggests that the (Si ) emission originates mostly from the ionized gas. The (Si )3 5µ mt o (N) 122 µm ratio indicates that silicon of 30% of the solar abundance must be in the diffuse ionized gas, suggesting that efficient dust destruction is undergoing in the ionized region.