It is known that the image quality of dichromated gelatin (DCG) phase hologram strongly depends on pre-hardening of the gelatin layer. In order to evaluate the relationship between pre-hardening and the holographic quality and to analyze its mechanism, eluted amount of gelatin from a DCG layer under developing conditions was determined and effect of exposure to the elution was examined. Taking into account the results of Cr (III) analysis in the previous study, the effect of the pre-hardening to the diffraction efficiency was discussed.