1. A crowd monitoring system that obtains a density of an object on the basis of image information, comprising: an image acquiring unit that acquires a plurality of images; an image feature quantity acquiring unit that obtains an image feature quantity of the object in the acquired image; a motion line acquiring unit that obtains a motion line of the object in the acquired image; a motion feature quantity acquiring unit that obtains a motion feature quantity of the object on the basis of the obtained motion line; a storage unit that stores information of a relation between the image feature quantity and the density of the object acquired in advance, and a relation between the motion feature quantity and the density of the object acquired in advance; and a density acquiring unit that determines the density of the object, wherein the density acquiring unit obtains a first density estimation value of the object on the basis of the obtained image feature quantity and a relation between the stored image feature quantity and the density of the object, and a second density estimation value of the object on the basis of the obtained motion feature quantity and a relation between the stored motion feature quantity and the density of the object.