1. An object detector comprising: a plurality of imaging sections that image a predetermined monitoring space; an image storage section that stores at least one of a plurality of images imaged by the plurality of imaging sections; a brightness change calculating section that calculates the amount of change in brightness between the stored preceding image and the present image; a three-dimensional information calculating section that calculates three-dimensional information from the plurality of images imaged by the plurality of imaging sections; a three-dimensional object detecting section that detects a three-dimensional object in the monitoring space based on the three-dimensional information calculated by the three-dimensional information calculating section; and an object candidate extracting section that extracts an object showing a large brightness change as a moving object candidate and an object showing a small brightness change as a still object candidate based on the change in brightness in the image area corresponding to the object detected by the three-dimensional object detecting section.