It aims to enable to uniform synthesized luminance in an image overlap region of a projection image by a simple process, by means of a mechanism of: performing display control to project a black image as the projection image to be projected from a first projection-type image displaying apparatus to a screen, and project, as the projection image to be projected from a second projection-type image displaying apparatus to the screen, an image in which dimming correction to the overlap region of the projection images respectively projected by the first and second projection-type image displaying apparatuses has been enabled; measuring a luminance characteristic of the second projection-type image displaying apparatus based on a photographed image obtained by photographing the overlap region; and setting a dimming correction characteristic for the overlap region by the first projection-type image displaying apparatus, based on the measured luminance characteristic.