The present invention relates to a process for efficiently amplifying a giant DNA. More particularly, the present invention relates to a process for amplifying DNA in a cell, comprising amplifying the DNA as the target of amplification in the presence of DNAs selected from the following (i), (ii) and (iii): (i) DNA encoding a protein selected from the following 1), 2) and 3): 1) a protein consisting of the amino acid sequence of SEQ ID NO: 1, 2) a protein comprising an amino acid sequence which has a deletion, substitution, insertion or addition of one or more amino acids in the amino acid sequence of SEQ ID NO: 1, and 3) a protein comprising an amino acid sequence which has an identity of 90% or more to the amino acid sequence of SEQ ID NO: 1, (ii) DNA consisting of the nucleotide sequence of SEQ ID NO: 2, and (iii) DNA hybridizing to the nucleotide sequence of SEQ ID NO: 2 under stringent conditions.