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, the captured image being acquired by a vehicle-mounted camera equipped with a wide-angle lens and being distorted; a setting unit that sets specific correction values for respective regions of the captured image that are separated by a reference line at a predetermined interval along a vertical direction of the captured image, the specific correction values being calculated based on a 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 horizontal direction to the captured image using the specific correction values 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.