1. An object detecting device comprising: an image acquiring unit configured to acquire an image from a camera; a scanning interval calculating unit configured to calculate a scanning interval when a detecting window is scanned on the image based on a size of a detection object on the image detected by the detecting window; a scanning unit configured to scan on the image using the scanning interval calculated by the scanning interval calculating unit; and a detecting unit configured to determine whether the detection object is present within the scanned detecting window.