The present invention provides an agent for improving plant growth which is highly safe and greatly improves not only resistance to temperature stress and dry stress but also resistance to a wide range of stresses to which plants can be exposed including chemical stress and UV stress. The agent for improving plant growth alleviates the environmental stress on plants promotes plant growth and improves quality. The main component of the agent is zerumbone or an analog thereof or a salt of those. Moreover the analog of zerumbone is characterized in being a humulene or caryophyllene.
