1. A focusing apparatus comprising: an image pickup device which obtains an image signal of a subject that is captured by an image capturing device, wherein the image capturing device incorporates a focus optical system therein; a search interval calculating device which acquires contrast characteristics that are obtained for a subject of a predetermined spatial frequency at a predetermined focal length and calculates a search interval in capturing of an image based on the contrast characteristics; a focus evaluation value acquiring device which moves a focus lens to detecting points in a predetermined searching range with the search interval therebetween and acquires a focus evaluation value at each detecting point from the image signal; and a focus detecting device which detects a focus position by using the focus evaluation value at each detecting point that is acquired by the focus evaluation value acquiring device.