1. An image superimposing apparatus comprising: an image-to-be-superimposed input means for inputting an image to be superimposed; a background image input means for inputting a background image which is used as a background against which the image to be superimposed is to be displayed; a display form input means for inputting a display form of the image to be superimposed; a display start position setting means for setting a display start position at which a display of the image to be superimposed on said background image is started; and a superimposing processing means for acquiring the image to be superimposed inputted by said superimposing image input means, the background image inputted by said background image input means, and the display form inputted by said display form input means, and for starting a display of said image to be superimposed from the display start position set by said display start position setting means, wherein said display start position corresponds to a deepest part of the background image which can be assumed to be located deepest with respect to a direction of a depth of the background image and wherein said superimposing processing means displays a flow of said image to be superimposed in said direction of the depth of the background image.