The ortho-isomer of molecular hydrogen (o-H-2) was converted to the para-isomer (p-H-2) within 600 s by using Prussian blue analogs, {M-3(II)[Cr-III(CN)(6)](2)} (MCr; M = Mn and Ni), as nuclear-spin conversion catalysts. The swift conversion was confirmed by in-situ Raman micro-spectroscopy under an H-2 gas atmosphere (100 kPa) in a low temperature range (20-90 K). The o-p ratio observed in MCr deviated from the theoretical value based on the Boltzmann distribution of H-2 in a free rotational state to the para-rich proportion, which suggested the promotion of the o-p conversion at higher temperature.