A three-dimensional object detection device comprising: an image capturing means mounted on a vehicle characterized by the image capturing means comprising a lens for capturing an image behind a host vehicle; a three-dimensional object detection means for detecting a three-dimensional object rearward of a host vehicle based on a captured image captured by the image capturing means; a first edge intensity calculation means that extracts the edges of a subject in a first edge extraction area that includes at least a horizon reference area that references a horizon and calculates an intensity of the edges in the first edge extraction area as a first edge intensity based on a distribution of the edges extracted from the first edge extraction area; a second edge intensity calculation means that extracts the edges of a subject in a second edge extraction area that includes at least a road edge reference area that references a road edge and calculates an intensity of the edges in the second edge extraction area as a second edge intensity based on a distribution of the edges that are extracted from the second edge extraction area; a day/night assessment means that assesses whether the present is daytime or nighttime; and a control means that controls the detection of the three-dimensional object by the three-dimensional object detection means based on the first edge intensity in the first edge extraction area when an assessment has been made that the present is daytime by the day/night assessment means and that controls the detection of the three-dimensional object by the three-dimensional object detection means based on the second edge intensity in the second edge extraction area when an assessment has been made that the present is nighttime. an image capturing means mounted on a vehicle characterized by the image capturing means comprising a lens for capturing an image behind a host vehicle; a three-dimensional object detection means for detecting a three-dimensional object rearward of a host vehicle based on a captured image captured by the image capturing means; a first edge intensity calculation means that extracts the edges of a subject in a first edge extraction area that includes at least a horizon reference area that references a horizon and calculates an intensity of the edges in the first edge extraction area as a first edge intensity based on a distribution of the edges extracted from the first edge extraction area; a second edge intensity calculation means that extracts the edges of a subject in a second edge extraction area that includes at least a road edge reference area that references a road edge and calculates an intensity of the edges in the second edge extraction area as a second edge intensity based on a distribution of the edges that are extracted from the second edge extraction area; a day/night assessment means that assesses whether the present is daytime or nighttime; and a control means that controls the detection of the three-dimensional object by the three-dimensional object detection means based on the first edge intensity in the first edge extraction area when an assessment has been made that the present is daytime by the day/night assessment means and that controls the detection of the three-dimensional object by the three-dimensional object detection means based on the second edge intensity in the second edge extraction area when an assessment has been made that the present is nighttime.