A plant growth promotion device includes: a flat-plate rotor, a motor coupled to the rotor by a rotary shaft, and a plurality of permanent magnets fixed in place at arbitrary intervals within the rotor, the rotor being rapidly rotatable by the rotary shaft that is coupled to the motor, so as to produce an alternating magnetic field. The device is used by situating and operating it in an agricultural field of crops so that the alternating magnetic field is produced in the agricultural field.
