The present invention provides a method for promoting plant growth, which comprises treating a plant with a compound represented by the following Formula (1): provided that a method for promoting plant growth which comprises treating plants with a compound corresponding to the following (1) or an agriculturally acceptable salt thereof are excluded, (1) benzo[b]thiophene-3-carboxylic acid. The method set forth in one of the above, the plant being soybeans. The method set forth in one of the above, the plant being cotton. A plant seed resulting from treatment with the compound of claim 1 containing an effective quantity of the compound indicated in formula (1). A composition for promoting plant growth and containing an inert component and the compound indicated in formula (1).
