The present invention provides a drone (unmanned aerial vehicle) capable of photographing a base part of the stem and a side of the leaf of the field crops for evaluating their growth status. A camera is positioned on the unmanned aerial vehicle such that the camera's field of view is directed backward with respect to the direction of the unmanned aerial vehicle. The camera captures an image of the crop temporarily knocked down by the downdraft created by the rotor of the drone, which exposes the base part of the stem and the side of the leaf to the sky. Optionally, the depression angle of the camera is automatically adjusted depending on the flight speed, wind force, and wind direction. Optionally, the camera body is automatically rotated to be directed to backward when the drone changes flying directions.