To obtain a new modified factor VIII cDNA having such a structure that the β-domain of a wild type factor VIII cDNA is deficient, and a partially cut-out factor IX intron 1 is introduced into a specific position of the factor VIII cDNA, and useful for improving the productivity of factor VIII, for gene therapy or the like.

This new modified factor VII cDNA has such a structure that the β-domain of a wild type factor VIII cDNA is deficient, and a partially cut-out factor IX intron 1 is introduced into 1 or more positions of the factor VIII cDNA. The cDNA is useful for producing factor VIII at high yield, or as a transfer vector of gene therapy or the like. It is obtained as follows: performing reverse transcription on a human cellular RNA isolated by the hepatobiopsy of a patient from whom informed consent is obtained, using a reverse transcriptase; synthesizing the cDNA; cloning the factor VIII cDNA whose β-domain is deficient through the synthesis of four PCR fragments whose βdomains are removed; and introducing a partially cut-out factor IX intron 1 into the obtained cDNA.