Evaluation of tree production capacity by a non-destructive method using a tree crown structure measurement device or using an image capture device with a fisheye lens mounted, for isolated trees or for trees existing at intervals, is enabled without requiring a large amount of time. A semi-spheroid of revolution model is established as a representation of the outer shape of a tree, an optical tree structure measurement device is placed at a position apart from the main trunk by a prescribed distance, the intensity of light passing through tree leaves and the intensity of light not passing through tree leaves are measured, the optical path length of light incident on the tree is calculated using the semi-spheroid model and the total tree leaf area is determined from the measured light intensities and optical path length. The total tree leaf area is also determined by placing an image capture device with a fisheye lens mounted in place of an optical tree structure measurement device and performing data processing of image data acquired by image capturing.
