PURPOSE: A reliability-based design optimization method is provided to improve convergence property without being affected by the problem characteristic by providing a most probable point renewal process using a conjugate gradients method. CONSTITUTION: A deterministic optimization that satisfies a constraint with minimizing the performance index is computed at the present design point (S110). A most probable point direction vector for computing a most probable point direction vector at the next design point is computed by using a sensitivity direction with respect to the deterministic optimization (S120). The most probable direction vector at the next design is computed based on the most probable point direction vector, and a reliability-based design optimization that satisfies a constraint with minimizing a performance index is computed (S130). If the reliability-based design optimization does not satisfy a convergence condition, the most probable point direction vector is computed by using a sensitivity direction or a conjugate gradient method with respect to the reliability-based design optimization (S150). [Reference numerals] (AA) Start; (BB) End; (S110) Perform a deterministic optimization; (S120) Calculate a direction vector for computing a most probable point direction vector with respect to the deterministic optimization; (S130) Perform an optimization based on reliability; (S140) Collect?; (S150) Calculate a direction vector for computing a most probable point direction vector with respect to the reliability-based optimization
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