In an optimum design method comprising a first solution determining step
of solving an optimization problem of a first evaluation function for a
state variable vector with a design variable vector being as a parameter,
and a second solution determining step of solving an optimization problem
of a second evaluation function for the design variable vector and the
state variable vector thus obtained, the second solution determining step
includes the steps of computing a gradient vector of the second
evaluation function for the design variable vector, computing a first
coefficient based on a value of a norm of the gradient vector, computing
a search vector based on the first coefficient, computing a second
coefficient, and updating the design variable vector based on the second
coefficient. The second coefficient computing step includes the first
solution determining step, the first solution determining step is
executed as an iterative method based on the gradient vector, and the
state variable vector is not initialized during iteration. The optimum
design method is precisely adaptable for structural changes.