1. A display apparatus comprising: a display unit configured to display an image having a part that an object is supposed to contact; a plurality of optical input units disposed in the display unit and configured to output electric signals in a magnitude corresponding to an amount of received light; a convert unit configured to receive the signals from each optical input unit and convert the signals into gradation values to obtain a picked-up image; a designation unit configured to designate an area in the picked-up image corresponding to the part; and a process unit configured to find a coordinate of the object in accordance with data within the designated area of the picked-up image.