PROBLEM TO BE SOLVED: To provide an agent for regulating the growth retarding of a seedling to control the growth of a plant to a prescribed delayed state. SOLUTION: The objective agent for controlling the growth of a plant sowed on a field to a prescribed delayed state contains an electrolytic strongly acidic water as an active component. The strongly acidic water is produced by the diaphragm electrolysis using an aqueous solution of an inorganic salt as the water to be electrolyzed.