1. An information processing apparatus comprising: an image acquiring computer configured to acquire captured image given by capturing an image of a watching target person whose behavior is watched and an image of a target object serving as a reference for a behavior of the watching target person, the captured image containing depth information indicating depths obtained on a pixel-by-pixel basis within the captured image; a foreground area extracting computer configured to extract a foreground area of the captured image on the basis of a difference between a foreground image and the captured image, the foreground image being set to contain a depth of the foreground as the foreground of the captured image; and a behavior presuming computer configured to determine a positional relationship between the foreground area and the area of the target object in a real space by determining relative position of the foreground area with respect to the target object and presume the behavior of the watching target person with respect to the target object by determining whether the positional relationship satisfies a predetermined condition or not on the basis of referring to the depths of the pixels within the foreground area on the basis of the depth information, the predetermined condition being set on the assumption that the extracted foreground area is related to the behavior of the watching target person, wherein when the watching target person conducts the behavior with respect to the target object, a range of a planar dimension of an area in which to obtain the depth of a predetermined region of the watching target person existing in the detection area in the real space, is set as the condition for presuming the behavior of the watching target person, the detection area set for the target object, and the behavior presuming computer is configured to: estimate a region area corresponding to the predetermined region of the watching target person in the field, included in the detection area, of the foreground area, calculate planar dimensions on a pixel-by-pixel basis in the real space in accordance with depths on the pixel-by-pixel basis in the region area indicated by the depth information, and presume the behavior of the watching target person with respect to the target object on the basis of determining whether a total sum of the planar dimensions calculated on the pixel-by-pixel basis in the region area is included in the planar dimension range set as the condition or not.