α-Ketol unsaturated fatty acid derivatives represented by the following formula (1), α-ketol unsaturated fatty acid amide derivatives represented by the following formula (6), and plant growth regulators containing, as the active ingredient, at least one of these derivatives: wherein R1 represents linear C1-5 alkyl; R represents hydrogen, etc.; R3 represents hydrogen, etc.; R4 represents hydrogen, etc.; R5 represents hydrogen, etc.; n is an integer from 3 to 15; and the carbon atoms at the A- and B-positions may be in either (R)- or (S)-configuration; and wherein R7 represents C1-17 alkyl, etc; R8 represents hydrogen, etc.; R9 represents α-ketolalkylene; and R10 represents specific divalent linear hydrocarbyl.