Abstract: A promotion device for carbon-dioxide assimilation of plant provided with a high-voltage generating portion which boosts voltage of electric current from an A/C power source portion and converts the electric current into direct current, and an electrode portion which impresses only minus potential of the current from the high-voltage generating portion. Potential difference between the minus potential impressed to the electrode portion and an earth is ?2000 to ?6000V. And, the electrode portion is inserted to soil around a root portion of the plant.