An image processing device comprising: a lane-width calculation unit, which calculates a width of a lane in which a vehicle travels from a captured image, in which a front of the vehicle in a traveling direction is captured, based on a correlation between a length of a subject in the captured image and a number of pixels constituting the captured image; a position information acquisition unit, which acquires information indicating a position of the vehicle; and a lane-width acquisition unit, which acquires information indicating a width of the lane in which the vehicle travels, from map data based on the information indicating the position of the vehicle, characterized by comprising: a storage unit, which stores the correlation between the length of the subject in the captured image and the number of pixels constituting the captured image; a scale correction unit, which corrects the correlation between the length of the subject in the captured image and the number of pixels constituting the captured image in a case where a difference between the width of the lane calculated by the lane-width calculation unit and the width of the lane acquired from the map data is greater than a predetermined value that serves as a criterion for correcting errors; and wherein the storage unit stores the corrected correlation between the length of the subject in the captured image and the number of pixels constituting the captured image, such that the width of the lane can be calculated based on the corrected correlation. a lane-width calculation unit, which calculates a width of a lane in which a vehicle travels from a captured image, in which a front of the vehicle in a traveling direction is captured, based on a correlation between a length of a subject in the captured image and a number of pixels constituting the captured image; a position information acquisition unit, which acquires information indicating a position of the vehicle; and a lane-width acquisition unit, which acquires information indicating a width of the lane in which the vehicle travels, from map data based on the information indicating the position of the vehicle, characterized by comprising: a storage unit, which stores the correlation between the length of the subject in the captured image and the number of pixels constituting the captured image; a scale correction unit, which corrects the correlation between the length of the subject in the captured image and the number of pixels constituting the captured image in a case where a difference between the width of the lane calculated by the lane-width calculation unit and the width of the lane acquired from the map data is greater than a predetermined value that serves as a criterion for correcting errors; and wherein the storage unit stores the corrected correlation between the length of the subject in the captured image and the number of pixels constituting the captured image, such that the width of the lane can be calculated based on the corrected correlation. a storage unit, which stores the correlation between the length of the subject in the captured image and the number of pixels constituting the captured image; a scale correction unit, which corrects the correlation between the length of the subject in the captured image and the number of pixels constituting the captured image in a case where a difference between the width of the lane calculated by the lane-width calculation unit and the width of the lane acquired from the map data is greater than a predetermined value that serves as a criterion for correcting errors; and wherein the storage unit stores the corrected correlation between the length of the subject in the captured image and the number of pixels constituting the captured image, such that the width of the lane can be calculated based on the corrected correlation.