The present invention provides an efficient synthetic method of &agr;-ketol unsaturated fatty acid having a double bond at a &bgr;-position to the ketone group thereof. It comprises the steps of: preparing compound (4) by reacting monosubstituted acetylene (2) with epoxide (3); and preparing &agr;-ketol unsaturated fatty acid (1) from said compound (4) as shown in Reaction Formula 1:
1
wherein R
1
represents an alkyl group of 1-18 carbon atoms or an aliphatic hydrocarbon group of 2-18 carbon atoms having 1-5 double or triple bonds at given positions; R
2
represents a protecting group for a hydroxyl group; R
3
represents a protecting group for a carboxyl group; R is identical to R
1
or, when R
1
has one or more triple bonds, represents an aliphatic hydrocarbon group in which each triple bond of R
1
is converted to a double bond; and A represents an alkylene group of 1-18 carbon atoms.