A motion detection device is provided. The motion detection device includes a first image recording unit, a first storage unit, a motion detection unit, a depth calculation unit, and a determination unit. The first image recording unit is configured to record a first video. The first storage unit is configured to store the first video. The motion detection unit is configured to detect a moving object in the first video. The depth calculation unit is configured to calculate a depth of the moving object. The determination unit is configured to determine whether or not the moving object is a concerned event according to the depth of the moving object.