1. A vehicle-mounted image processing apparatus comprising a processor configured to: acquire a captured image of one or more markers set in a marker settable range; detect: a plurality of edges of the one or more markers in the image or a plurality of feature points of the one or more markers in the image, each of the plurality of feature points indicating an end point of one of the one or more markers or a center of gravity of one of the one or more markers; estimate a plurality of straight lines based on the plurality of edges or the plurality of feature points and generates a virtual polygon region surrounded by the plurality of straight lines and a plurality of vertexes each being an intersection point of the plurality of straight lines; and calculate a camera parameter based on a feature value of the virtual polygon region in the image and a feature value of the virtual polygon region in a real space, wherein: the feature value of the virtual polygon region in the image is information on a shape of the virtual polygon region in the image, the feature value of the virtual polygon region in the real space is information on a shape of the virtual polygon region in the real space, and at least one of the plurality of vertexes is a virtual vertex which is included in a region outside the marker settable range and is different from the edge or the feature point in the image.