The plant growth promoting device includes a vessel for introducing a root of a plant into a culture solution of a cultivated plant, a circulation device for circulating the culture solution, a power source for applying a voltage, and at least the counter electrode, In which a voltage is applied and opposed to each other； and a control device that controls the application of the voltage. The counter electrode has no movable part in the part which is in contact with the root of the plant, and at least one of the counter electrodes applies a voltage while sequentially changing the application area in the direction of the flow of water in which the culture solution is circulated by the circulation device.
