A three-dimensional object detection device has an image capturing unit, an object detection unit, first and second edge intensity calculation units, a day/night assessment unit and a controller. The day/night assessment unit assess whether it is currently daytime or nighttime when detecting a three-dimensional object based on the captured images. Upon assesses it is daytime, edges of a subject are extracted from a first edge extraction area, including a horizon reference area, and a threshold value for detecting the three-dimensional object is set based on the intensity of the edges in the first edge extraction area. Upon assesses it is nighttime, the edges of a subject are extracted from a second edge extraction area, including a road edge reference area, and a threshold value for detecting the three-dimensional object is set based on the intensity of the edges that are extracted from the second edge extraction area.