The invention provides a lactone derivative of formula (I) which exhibits excellent rooting activity, and a plant growth regulator containing the derivative as an active ingredient. <CHEM> wherein R<1> and R<2> each individually represent a hydrogen atom, a halogen atom, an alkyl group having 1 to 6 carbon atoms, or an alkoxyl group having 1 to 6 carbon atoms; A represents a methylene group, a vinylene group, an imino group, an oxygen atom, or a sulfur atom; and n is an integer of 1 to 4; wherein when A is an imino group, both of R<1> and R<2> are atoms or groups other than hydrogen, and when A is an oxygen atom, both of R<1> and R<2> are hydrogen atoms.
