The diffusion coefficient of hydrogen in iron with various interstitial impurity (C, N) contents has been measured at 298 K by an electrochemical permeation method. The diffusivity of hydrogen is significantly reduced by cold-working. In commercially pure iron specimens (high C, N specimens), the diffusivity of hydrogen is slightly restored by aging at room temperature after cold-working. In decarburized-denitrided specimens (low C, N specimens), however, this change by aging is not observed. Annihilation of dislocations by heat treatments causes recovery of the diffusion coefficient. That is, dislocations serve as trapping sites for hydrogen, and the formation of atmosphere of interstitial impurities (C, N) around dislocations weakens the trapping effect of dislocations. The diffusion coefficient of hydrogen has been analyzed on the basis of the trapping theory, and the trap density (i.e., the ratio of the number of the trapping sites to that of total lattice sites) has been calculated. The trap density before the formation of the C, N atmosphere around dislocations is, within experimental error, proportional to the dislocation density.