to the function to extract the image of the object 603 and to calculate the position of the extracted image of the object 603 on the screen the image extracting circuit 601 provides position data indicating the position of the image of the object on the screen and gives a light detection signal i e a click signal when an image of the light emitted by the light emitting element 801 is displayed on the screen the microcomputer 602 converts the signal format of the light detection signal into a signal format that can be dealt with by the computer apparatus 109 i e a same signal format as that of the output signal of a mouse 301 provided when the click button of the mouse 301 is pressed the microcomputer 602 sends the click signal through the signal line 108 connected to the i o port thereof to the computer apparatus 109 the video camera 600 of the computer system shown in fig6 similarly to that of the computer system shown in fig1 gives digital video signals to the computer apparatus 109 and hence can be used also as one of common image data supply sources for the computer apparatus the computer system illustrated in fig6 to 9 like the computer system illustrated in fig1 to 5.