An object of the present invention is to provide a transformation system for Labyrinthulomycota that allows the elucidation of biosynthetic mechanisms of lipids such as PUFA and carotenoids as well as for the construction of a high production system and the design and development of novel functional lipid molecules by the control of the mechanisms. The present invention provides a method for introducing a transgene into a cell of Labyrinthulomycota, which comprises introducing into a cell of Labyrinthulomycota a recombinant vector comprising a transgene and a nucleotide sequence which is homologous to a part of chromosomal DNA of Labyrinthulomycota and is capable of homologous recombination with the chromosomal DNA, and then inducing homologous recombination in this homologous nucleotide sequence.