An image measuring system includes: a reflecting body arranged near a plant to be measured; a photographing unit for photographing the reflecting body and outputting the photographed image; an image analyzing unit for detecting, in the photographed image, the area of the reflecting body and acquiring the color information of the area; an absolute spectral irradiance distribution calculating unit for calculating, on the basis of the color information, the absolute spectral irradiance distribution, which is the distribution of the absolute values of the spectral irradiance in the area; a photon flux density calculating unit for calculating, on the basis of the absolute spectral irradiance distribution, the photon flux density; and a Leaf Area Index calculating unit for calculating, on the basis of the photon flux density, the Leaf Area Index.