For die-casting, various approaches to finding the optimal control settings and optimal mold by using a computational fluid dynamics (CFD) simulator have recently been studied and applied. However, the optimal value obtained by using a conventional CFD simulator does not consistently agree with experiment, owing to computational error. Accordingly, we analyze the problem of optimization with a CFD simulator in order to develop an optimization method that can consistently produce suitable results. To assess the effectiveness of the proposed method, it was applied to the optimization of plunger speed in die-casting.