PURPOSE: To generate efficiently and in a short time an octo-tree by providing a memory for storing the octo-tree, and using a command for displaying a graphic based on this memory.

CONSTITUTION: In a register 1, the present travel list is saved, and in a register 2, the previous travel list is saved. Also, in a register 3, a current path list is saved. In a memory 5, an octo-tree is stored. A central procesing unit 4 generates the octo-tree, whenever an element of a two-dimensional array is sent from a host computer (not shown in the figure). It is stored in the memory 5, also a command for displaying a graphic based on this octo-tree, and the command is sent to a display control part 6. The display control part 6 generates a display data based on a command from the central processing unit 4 and gives it to a display 7.