A three-dimensional object detection device (1, 1a) for detecting an adjacent vehicle whose headlights are on, comprising: an image capturing means provided with a lens for forming an image of a view rearward of a host vehicle; and a first three-dimensional object detection means (33, 33a) for detecting a three-dimensional object based on a captured image obtained by the image capturing means, characterized by further comprising: a light source detection means for detecting a light source which is present behind the host vehicle based on the captured image obtained by the image capturing means; a second three-dimensional object detection means for detecting a three-dimensional object based on the light source detected by the light source detection means; a degree-of-clouding calculation means (36, 36a) for calculating the degree to which the lens is clouded as the degree of lens clouding based on a plurality of sequentially captured images; and a control means for imparting weighting (Wc1, Wc2) in accordance with the degree of lens clouding to the detection result from the first three-dimensional object detection means (33, 33a) and to the detection result from the second three-dimensional object detection means, making the weighting of the detection result from the first three-dimensional object detection means (33, 33a) smaller and the weighting of the detection result from the second three-dimensional object detection means larger the higher the degree of lens clouding is, and assessing whether the three-dimensional object is an adjacent vehicle which is present in an adjacent lane adjacent to a lane of travel of the host vehicle based on the sum of the weighted detection results from the first three-dimensional object detection means (33, 33a) and from the second three-dimensional object detection means. an image capturing means provided with a lens for forming an image of a view rearward of a host vehicle; and a first three-dimensional object detection means (33, 33a) for detecting a three-dimensional object based on a captured image obtained by the image capturing means, characterized by further comprising: a light source detection means for detecting a light source which is present behind the host vehicle based on the captured image obtained by the image capturing means; a second three-dimensional object detection means for detecting a three-dimensional object based on the light source detected by the light source detection means; a degree-of-clouding calculation means (36, 36a) for calculating the degree to which the lens is clouded as the degree of lens clouding based on a plurality of sequentially captured images; and a control means for imparting weighting (Wc1, Wc2) in accordance with the degree of lens clouding to the detection result from the first three-dimensional object detection means (33, 33a) and to the detection result from the second three-dimensional object detection means, making the weighting of the detection result from the first three-dimensional object detection means (33, 33a) smaller and the weighting of the detection result from the second three-dimensional object detection means larger the higher the degree of lens clouding is, and assessing whether the three-dimensional object is an adjacent vehicle which is present in an adjacent lane adjacent to a lane of travel of the host vehicle based on the sum of the weighted detection results from the first three-dimensional object detection means (33, 33a) and from the second three-dimensional object detection means. a light source detection means for detecting a light source which is present behind the host vehicle based on the captured image obtained by the image capturing means; a second three-dimensional object detection means for detecting a three-dimensional object based on the light source detected by the light source detection means; a degree-of-clouding calculation means (36, 36a) for calculating the degree to which the lens is clouded as the degree of lens clouding based on a plurality of sequentially captured images; and a control means for imparting weighting (Wc1, Wc2) in accordance with the degree of lens clouding to the detection result from the first three-dimensional object detection means (33, 33a) and to the detection result from the second three-dimensional object detection means, making the weighting of the detection result from the first three-dimensional object detection means (33, 33a) smaller and the weighting of the detection result from the second three-dimensional object detection means larger the higher the degree of lens clouding is, and assessing whether the three-dimensional object is an adjacent vehicle which is present in an adjacent lane adjacent to a lane of travel of the host vehicle based on the sum of the weighted detection results from the first three-dimensional object detection means (33, 33a) and from the second three-dimensional object detection means.