This invention is intended to be used to search for a transcription factor having novel functions of increasing the weight of an individual plant, increasing the weight of a given tissue per individual plant, or improving the productivity of a given substance per individual plant and to improve such properties in the plant. The weight of an individual plant is increased, the weight of a given tissue per individual plant is increased, the productivity of a given substance per individual plant is improved, or the content of a given substance per given tissue of a plant is increased via expression of a transcription factor that has been modified to suppress transcription accelerating activity.