Genome editing using the CRISPR/Cas system can induce mutations with high efficiency, and allows easier production of genome-modified animals than that offered by the conventional method where embryonic stem cells are used. However, studies using CRISPR/Cas systems have been mostly limited to proliferating somatic cells and pronuclear-stage fertilized eggs. In contrast, the efficiency of a CRISPR/Cas system in immature and maturing oocytes progressing through meiosis has not yet been assessed. In the present study, we evaluated the genome-modification efficiency of the CRISPR/Cas system during meiotic maturation of porcine oocytes. Additionally, the localization of the Cas9 protein in immature oocytes was analyzed in relation to nuclear transport and mutation induction. The results showed that CRISPR/Cas induced mutation with high efficiency even in maturing oocytes with condensed chromosomes, whereas mutations were not induced in GV-stage oocytes. The localization analysis of enhanced green fluorescent protein (EGFP)-tagged Cas9 (Cas9-EGFP) revealed that the nuclei contained lesser Cas9 than the cytoplasm in immature oocytes. Treatment with leptomycin B, a nuclear export inhibitor, increased the amount of nuclear Cas9 and enabled mutation induction in GV oocytes. Our results suggest that CRISPR/Cas systems can be applied to oocytes during meiotic maturation and be implemented in novel applications targeting female genomes.