1. A method for detecting crowd density, the method comprising: projecting a depth image obtained by photographing onto a height-top-view, the depth image including a crowd; dividing the height-top-view into cells with a predetermined size; for each cell, extracting a density detection feature indicating distribution of differences in height between pixels in the cell; and detecting, based on the density detection feature, using a density model previously constructed by a statistical learning method, number of persons in each of the cells.