A process for producing 7-dehydrocholesterol, 7DHC, comprising: culturing, in a medium, a 7DHC-producing Labyrinthulea microorganism in which the 7DHC reducing activity is reduced or lost as compared to a parent strain through deletion, substitution, or addition of at least one base in a gene which is present in the chromosomal DNA of the parent strain and encodes a protein having 7DHC reducing activity, and the microorganism produces 7DHC; allowing 7DHC to be produced and accumulated in the culture; and collecting the 7DHC from the culture, wherein the gene encoding the protein having 7DHC reducing activity is a gene having any of the following DNAs to: a DNA encoding a protein having the amino acid sequence represented by SEQ ID NO: 2; a DNA encoding a mutated protein consisting of the amino acid sequence of SEQ ID NO: 2 with deletion, substitution, or addition of 1 to 20 amino acids, and having 7DHC reducing activity; a DNA encoding a homologous protein having at least 95% identity with the amino acid sequence represented by SEQ ID NO: 2, and having 7DHC reducing activity; a DNA having the base sequence represented by SEQ ID NO: 1; a DNA that hybridizes with DNA consisting of a base sequence complementary to the base sequence represented by SEQ ID NO: 1 under stringent conditions, and encodes a homologous protein having 7DHC reducing activity; and a DNA having at least 95% identity with the base sequence represented by SEQ ID NO: 1, and encoding a homologous protein having 7DHC reducing activity. culturing, in a medium, a 7DHC-producing Labyrinthulea microorganism in which the 7DHC reducing activity is reduced or lost as compared to a parent strain through deletion, substitution, or addition of at least one base in a gene which is present in the chromosomal DNA of the parent strain and encodes a protein having 7DHC reducing activity, and the microorganism produces 7DHC; allowing 7DHC to be produced and accumulated in the culture; and collecting the 7DHC from the culture, wherein the gene encoding the protein having 7DHC reducing activity is a gene having any of the following DNAs to: a DNA encoding a protein having the amino acid sequence represented by SEQ ID NO: 2; a DNA encoding a mutated protein consisting of the amino acid sequence of SEQ ID NO: 2 with deletion, substitution, or addition of 1 to 20 amino acids, and having 7DHC reducing activity; a DNA encoding a homologous protein having at least 95% identity with the amino acid sequence represented by SEQ ID NO: 2, and having 7DHC reducing activity; a DNA having the base sequence represented by SEQ ID NO: 1; a DNA that hybridizes with DNA consisting of a base sequence complementary to the base sequence represented by SEQ ID NO: 1 under stringent conditions, and encodes a homologous protein having 7DHC reducing activity; and a DNA having at least 95% identity with the base sequence represented by SEQ ID NO: 1, and encoding a homologous protein having 7DHC reducing activity. a DNA encoding a protein having the amino acid sequence represented by SEQ ID NO: 2; a DNA encoding a mutated protein consisting of the amino acid sequence of SEQ ID NO: 2 with deletion, substitution, or addition of 1 to 20 amino acids, and having 7DHC reducing activity; a DNA encoding a homologous protein having at least 95% identity with the amino acid sequence represented by SEQ ID NO: 2, and having 7DHC reducing activity; a DNA having the base sequence represented by SEQ ID NO: 1; a DNA that hybridizes with DNA consisting of a base sequence complementary to the base sequence represented by SEQ ID NO: 1 under stringent conditions, and encodes a homologous protein having 7DHC reducing activity; and a DNA having at least 95% identity with the base sequence represented by SEQ ID NO: 1, and encoding a homologous protein having 7DHC reducing activity.