A display device (1) is placed on a document table (101) of a copy machine (100) such that a reading area of the copy machine (100) and a display screen (10) of the display device (1) face each other. Then, the position of the display device (1) is adjusted such that a rear surface identification mark (11), which is a straight line and indicates the position of a short side of the display screen (10), and a short side reference line (107), which is a short side of the document table (101), are connected by a straight line, and such that a rear surface identification mark (12), which is a straight line and indicates the position of a long side of the display screen (10), and a long side reference line (108), which is a long side of the document table (101), are connected by a straight line.