We investigate the characteristics of far-infrared brightness fluctuations at 90 μm and 170 μm in the Lockman Hole which were surveyed with ISO, using power spectrum analysis over the spatial frequency range of 0.05... 1 arcmin-1. The power spectra are found to be rather flat at low spatial frequencies (f ≤ 0.1 arcmin-1), and tend to decrease slowly towards the higher frequencies. The latter—unlike the power-law ones—are seen in the IR cirrus fluctuations, and are well explained by randomly distributed point sources. We conclude that the fluctuations due to the IR cirrus are not dominant in the Lockman Hole and are most likely due to faint galaxies. We also give constraints on the galaxy number counts, which indicate the existence of strong evolution in the counts.