According to the present invention, there can be provided A process for producing 7-dehydrocholesterol (hereinafter, “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; and a process for producing vitamin D3, comprising irradiating, with ultraviolet light, the 7DHC produced by the production process.