1. An image generation device for monitoring surroundings of a vehicle, comprising: an input unit to which a captured image of the surroundings of the vehicle is input as a distortion-uncorrected captured image, including distortion in a vertical direction and distortion in a horizontal direction, the captured image being acquired by at least one vehicle-mounted camera equipped with a wide-angle lens and being distorted, the captured image including a plurality of regions; a setting unit that sets a specific correction value for each of the regions of the captured image that are separated by a reference line at a predetermined interval along the vertical direction of the captured image, the specific correction value being calculated based on a predetermined reference correction value of the wide-angle lens along the vertical direction of the captured image, the reference line extending in a horizontal direction of the captured image; an execution unit that applies distortion correction in only the vertical direction and not in the horizontal direction to each of the regions of the captured image using the specific correction value for the respective region set by the setting unit; and an output unit that outputs, to a monitor, the captured image to which the distortion correction has been applied by the execution unit.