1. An image processing method comprising: a physical image acquisition step of acquiring an image of a physical space; a viewpoint position and orientation acquisition step of acquiring a position and orientation of a viewpoint of the image of the physical space; a generation step of generating an image of a virtual object, based on the acquired position and orientation; a detection step of detecting, as an object area, an area which consists of pixels each having a predetermined pixel value, from the image of the physical space; and a superimposition step of superimposing the image of the virtual object on the image of the physical space, wherein the superimposition step includes: a calculation step of calculating a distance between a position of the virtual object and a position of the viewpoint; an acquisition step of acquiring, from an operation panel operated by an operator, an operator instruction indicating whether or not the virtual object is to be emphasis-displayed; a setting step of changing a flag to indicate that the image of the virtual object is to be set as a masked target when both: a) the operator instruction indicates that the virtual object is not to be emphasis-displayed, b) the calculated distance is not smaller than a predetermined distance, and changing the flag to indicate that the image of the virtual object is not to be set as a masked target when both: a) the operator instruction indicates that the virtual object is to be emphasis-displayed, b) the calculated distance is smaller than the predetermined distance; a step of, while the flag is set to indicate that the image of the virtual object is to be set as the masked target, applying a masking process to a portion of the image of the virtual object which overlaps with the object area, and superimposing the masking processed image of the virtual object on the image of the physical space; and a step of, while the flag is set to indicate that the image of the virtual object is not to be set as the masked target, without applying a masking process to the portion of the image of the virtual object, superimposing the image of the virtual object on the image of the physical space.