An inducing agent for plant root including a ketol unsaturated fatty acid having 5 to 24 carbon atoms, 1 to 6 double bonds between carbon atoms and an α ketol structure or γ ketol structure (in particular 9-hydroxy-10-oxo-12( Z ), 15( Z )-octadecadienoic acid is preferable), which inducing agent for plant root may be used to promote or induce the root growth even when transplanting by cutting plants for which root generation is hard such as pine, cedar, tea, chestnut, may be used even for root growth of cuttings of Prunus x yedoensis (cherry trees) for which root growth is considered impossible, Hypericum chinense for which is transplantation by cutting is said to be difficult, and Paraserianthes falcataria Becker useful as a material for plywood, and may be used by a simple technique such as spraying.