1. A traffic monitoring apparatus which comprises: an object information acquiring device for acquiring three-dimensional object information related with an object present in an area under surveillance; a first information generating device for generating a first quantitative information related with the number of objects based on at least a volume, which is obtained from the object information including volume data and a height of an object; a second information generating device for generating a second quantitative information related with the number of objects based on a distribution of the volumes of objects in a two-dimensional plane from the object information; and a determining device for determining the number of the objects based on the first and second quantitative information, wherein the object information acquiring device includes a block volume calculating unit for determining, for each of a plurality of blocks into which the area of surveillance is divided, a height of an object in each block and for calculating a volume based on the determined height of the object in each block.