PURPOSE: To monitor the extent of the growth of a plant directly without exerting any adverse influence upon the growth by contacting the plant by analyzing the extent of the growth of the plant stem from time variation in the distance from the top of a ring arranged nearby the plant stem to the surface of the plant stem, and displaying the extent of the growth.
CONSTITUTION: A ring-shaped frame 1 clamps the outer periphery of the plant stem 7 to be monitored at a constant-distance interval. An actuator 2 is movable on the ring-shaped frame 1. Further, an optical system 3 and a photosensor part 4 constitute a position detecting device. Then a data processor 5 receives and analyzes data on the distance to the plant stem 7 which is measured by the position detecting device and the sectional shape of the stem 7 which is analyzed from the distance to the plant stem 7 measured by moving the actuator 2 on the ring-shaped frame 1 is displayed on a display device 6 at constant intervals of time.