1. A mobile detector, comprising: a virtual plane image generating device which generates, on an assumption that each pixel of an input image obtained by an imaging device on a mobile corresponds to each point on a virtual plane on a real world parallel to a traveling direction of the mobile, a virtual plane image by converting the input image to an image viewed from a direction perpendicular to the virtual plane; and a mobile detecting device which relates each of points on two virtual plane images which are generated at two different times, obtains a shift amount between corresponding points, and detects a detection object mobile based on an arrangement of the points having the same shift amount, wherein the mobile detecting device determines a presence of the detection object mobile when the number of points having the same shift amount in a partial area of the virtual plane image is equal to or more than a predetermined threshold value.