Abstract: To provide a transformation method for producing a stramenopile organism having an improved unsaturated fatty acid production capability by disrupting a gene of the stramenopile organism or inhibiting the expression of the gene in a genetically engineering manner. [Solution] A method for transforming a stramenopile organism, which comprises disrupting a gene of the stramenopile organism or inhibiting the expression of the gene in a genetically engineering manner, and which is characterized in that the stramenopile organism is selected from Thraustochytrium aureum, Parietichytrium sarkarianum, Thraustochytrium roseum and Parietichytrium sp. and the gene to be disrupted or of which the expression is to be inhibited is a gene associated with the biosynthesis of a fatty acid.