PURPOSE: To accurately form a design back-up system by verifying a design plan proposed by a proposing mechanism via a verification function and at the same time generating a desired plan.

CONSTITUTION: A proposing mechanism 2 applying an inferring function proposes a design plan based on the knowledge of a designer, and an inspecting mechanism 3 evaluate the design plan. Then a searching mechanism 1 which controls both mechanisms 2 and 3 compares the design plans with each other and generates an optimum design through the study. Thus it is possible to perform a designing operation close to a human designing method which repeats the proposal and the verification via a computer. In such a constitution, the correct design result is outputted despite the presence of the ambiguous or wrong knowledge.