1. An image processing apparatus that communicates with an image acquisition apparatus provided with an image acquisition region comprising both a light-shielded region where a plurality of light-shielded solid-state image acquisition elements are arrayed as light-shielded pixels and a non-light-shielded region where a plurality of exposed solid-state image acquisition elements are arrayed two-dimensionally as effective pixels for acquiring an image of an object, the image processing apparatus comprising: image data acquiring means for acquiring data of the image based on output signals from the effective pixels which are provided by the solid-state image acquisition elements in the non-light-shielded region of the image acquisition apparatus; edge extracting means for extracting an edge in the image, the is edge being composed of pixels whose values differ from values of adjacent pixels in the effective pixels by differences each being equal to or larger than a preset edge threshold; object recognizing means for recognizing the object based on the extracted edge; output acquiring means for acquiring output signals from the light-shielded pixels which are provided by the solid-state image acquisition elements in the light-shielded region; noise estimating means for estimating a degree of variations in noise contained in the output signals from the effective pixels, based on the output signals acquired by the output acquiring means; and threshold setting means for setting the edge threshold based on the degree of variations in noise which is estimated by the noise estimating means, such that the noise having a level which exceeds the edge threshold occurs at a probability lower than a preset value.