Abstract: An adjusting screen with a predetermined format including a specific indication mark which is used to correct the display irregularity of the screen in a relationship between a plurality of mirrors and regions of the screen with a predetermined format is generated to be displayed on a display screen. In accordance with a shift input instruction that moves the screen which is displayed on the display screen in a forward or backward direction as seen from a user, the screen shift control is performed to control the adjusting screen including a specific indication mark to be moved forward or backward by a predetermined amount to be displayed on the display screen.