PURPOSE: To easily and surely indicate an object displayed on the most operator' s side of a display device by providing the three-dimensional computer graphic system with a recognizing device having a specific function.

CONSTITUTION: The coordinate data of a picture element position on the screen of the display device 20 are inputted to the recognizing device 40 by the instruction of a mouse 30. The device 40 stores the object name (identifier) of an object plotted on an indication point in a variable O and substitutes the Z value (depth buffer value) of a Z buffer 12 corresponding to specified coordinates for a variable Z1. The C-th order object is plotted, a Z value corresponding to the coordinates of the picture element is inquired of the Z buffer 12 and a return value Z is compared with the variable Z1. When the C-th plotted object is displayed on the most operator's side out of plural plotted objects, the object name of the C-th object is set up in the variable O and the Z value of the C-th object is inputted to the variable Z1. This processing is repeated only by the total number of objects. Consequently object displayed on the display screen can be specified by th observed sense.