1. An individual detector, comprising: a range image sensor that is disposed to face downward to a detection area below and generates a range image, each image element of the range image including, when one or more physical objects exist in said area, each distance value up to the one or more physical objects, respectively; and an object detection stage that separately detects the one or more physical objects in said area based on the range image generated with said sensor, wherein said object detection stage generates a foreground range image by extracting a specific image element from each image element of a present range image, the specific image element being extracted when a distance differential is larger than a prescribed distance threshold value, the distance differential being obtained by subtracting an image element of said present range image from a corresponding image element of a background range image, wherein the background range image is a range image previously obtained from said sensor, wherein said object detection stage separately detects one or more persons as the one or more physical objects to be detected in said area based on the foreground range image, wherein said object detection stage detects one or more physical objects to be detected in said area based on data of part of a specific, or each, altitude of the one or more physical objects to be detected, the data being obtained from said range image, generates a foreground range image by extracting a specific image element from each image element of said range image, and separately detects one or more persons as one or more physical objects to be detected in said area based on the foreground range image, and said specific image element is extracted when a distance value of an image element of said range image is smaller than a prescribed distance threshold value.