1. An apparatus mounted on a vehicle for monitoring vicinity of the vehicle, comprising: imaging means for capturing an image of the vicinity of the vehicle; extracting means for extracting an object candidate from the captured image; means for calculating distance (Z 0 ) from the vehicle to the object candidate in the current image and distance (Z 1 ) from the vehicle to the object candidate in the previous image; means for producing a virtual object by decreasing the size of the object candidate in the current image by a ratio of the distance to the object candidate in the current image and in the previous image or by extending the size of the object candidate in the previous image by the ratio of the distance to the object candidate in the previous image and in the current image; comparing means for comparing the size (Ghv) of the virtual object with the size (Ghd 1 ) of the object candidate in the previous image or with the size (Ghd 0 ) of the object candidate in the current image; and means for determining that the extracted object candidate is an object of the desired category if the comparison substantially matches.