This disclosure provides an agent for improving plant growth, the agent significantly improving resistances to a wide variety of stress possibly subjected to a plant, such as resistances to chemical stress and ultraviolet ray stress as well as resistances to temperature stress and dry stress, the agent having a high safety. This agent for improving plant growth alleviates an environmental stress on a plant, promotes plant growth, or a plant quality. This agent for improving plant growth contains, as its main ingredient, zerumbone, an analog of zerumbone, or a salt of zerumbone or the analog. Furthermore, in this agent for improving plant growth, the analog of zerumbone is α-humulene or β-caryophyllene.
