An electronic blackboard displays information and optically detects input information on a display/writing surface by determining the corresponding coordinates. To determine the input coordinates, a predetermined number of pairs of light emitting elements and light detecting elements are placed in parallel for each axis near the edges. The light emitting elements and the light detecting elements are also linearly and equidistantly placed. A control unit is connected to the light emitting elements for sequentially activating these light emitting elements and for controlling the light detecting elements to detect the light in an overlapping manner. Based upon the above positional information from the light detecting elements, an input coordinate determination unit determines the input coordinates of an input on the display/writing surface.