PURPOSE: To save the labor for the block diagram updating by interlocking the device information and inter-device connection information of a conventionally independent system and the block diagram displaying of the system with the graphic displaying rules.

CONSTITUTION: A block diagram display to display the block diagram of a computer application system composed of plural devices by the combination of a graphic is constituted of a constitution control data base 1 to record the device information to show the corresponding relation of a graphic and a device and the inter-device connection information to show the corresponding relation of the graphic and the inter-device connection, a displaying rule data base 2 to store the graphic displaying rules, an arrangement deciding part 3 to arrange the graphic in accordance with the graphic displaying rules, and a displaying part 4 to display the block diagram by the instruction of the arrangement deciding part 3. As this result, based on the device information and the inter-device connection information recorded in the constitution control data base 1, by using the graphic displaying rules recorded in the displaying rule data base 2, the arrangement deciding part 3 decides the graphic expression of the configuration of the system and the block diagram displaying is instructed at the displaying part 4.