A conjugated fatty acid such as the cis-9, trans-11 isomer of conjugated linoleic acid having high bioactivities is selectively and efficiently produced by conjugating an unsaturated fatty acid having at least two double bonds with the use of viral cells, dead cells or a cell extract of at least one bacterium selected from the group consisting of Lactobacillus oris, Lactobacillus pontis, Lactobacillus panis, Bifidobacterium breve, Bifidobacterium bifidum, Bifidobacterium infantis and Bifidobacterium pseudocatenulatum, or an enzyme derived from the bacterium.