PURPOSE: To input a designated picture element to a computer while being confirmed with an operator, by detecting picture element segment passing within a predetermined interval to a cursor symbol and changing the luminance.

CONSTITUTION: Coordinates of X4 and Y4 coordinate counters 6 and 7 of a cursor symbol 3 are changed by operating a track ball 5. A computer 18 periodically reads them and displays the cursor symbol 3 on a screen 4 of a CRT1. Start and end point coordinates X1, Y1 and X2, Y2 of a picture element 1 are set to registers 10W13, and a linear equation forming circuit 14 forms an equation of straight lines of both the points. A residual calculating circuit 15 substitutes the values of the coordinates X4 and Y4 to an equation formed with the circuit 14 to obtain the residual of the equation. When the residual is smaller than a specified value, the circuit 15 controls a luminance control circuit 16 and dipslays the picture element 1 displayed on a CRT19 in high luminance. Thus, the operator can input the designated picture element to the computer while confirming the element.