1. An image signal processing apparatus comprising: an operation device which accepts operational instructions for changing an image quality parameter of an inputted image signal; an image signal generation circuit which generates an image signal for displaying at least two identical images based on the inputted image signal on one image plane; an image quality adjusting circuit which performs an image quality adjusting processing based on an image quality parameter before adjusting image quality with respect to a part of the generated image signal for displaying one of the identical images, and performs an image quality adjusting processing based on an image quality parameter after adjusting image quality with respect to another part of the generated image signal for displaying another one of the identical images, in response to the operational instructions for changing the image quality parameter accepted by the operation device.