To more reduce processing load in detection of an object by using a distance image, while reducing the amount of information keeping information for determining whether it is the object or not.

A distance image is created by an active-type distance image sensor 1. A background acquisition means 2 stores a background distance image, and a difference image creation means 3 creates a distance difference image from the distance image and the background distance image. A pixel-of-interest extraction means 4 extracts pixels whose pixel values in the distance difference image are a presence threshold or above from the distance image as pixels of interest, and a density measuring means 5 maps the pixels of interest in a three-dimensional virtual space, and finds the number of pixels of interest in each block of a unit volume as a density. An effective block extraction means 6 extracts blocks whose densities are an object threshold or above as effective blocks, and a region integration means 7 integrates adjoining effective blocks and forms a group. An object determination means 8 determines the group as an object, if the size of the group is in a proper range.