Abstract: The purpose of the present invention is to provide a technique by which a site-specific homozygote can be efficiently acquired by increasing cross efficiency and a homozygote in a genome modification-free state can be easily obtained. A method for producing homozygous cells, said method including: (A) a step for, into cells having a heterozygous mutation at a target site, introducing heterozygouse allele specific cleavage of DNA double strand of homologous chromosomes and thus inducing crossing in the presence of a Bloom's syndrome protein inhibitor to thereby give homozygous cells at the target site; and (B) a step for selecting the homozygous cells.